fixed GetPages.curve

- fixes #465
- iOS has an exception to the rule, when no Curve is defined, it doesn't use linearTransition in `CupertinoPages` (`Transition.cupertino/native/default`) and fullscreen dialogs so it applies the Curve, or use the default one.
It might be smarter, maybe, to also enforce the `animation` duration to stay in platforms defaults.
- had to change `CupertinoTransitions.buildTransitions` params in order to make the check and keep the code change small.

 import 'dart:math';
 import 'dart:ui' show lerpDouble;
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
@@ -7,6 +8,7 @@ import 'package:get/route_manager.dart';
 import 'package:get/src/core/get_main.dart';
 import 'package:get/src/instance/get_instance.dart';
 import 'package:get/utils.dart';
+
 import 'bindings_interface.dart';
 import 'custom_transition.dart';
 import 'default_transitions.dart';
@@ -136,18 +138,24 @@ class GetPageRoute<T> extends PageRoute<T> {
   @override
   Widget buildTransitions(BuildContext context, Animation<double> animation,
       Animation<double> secondaryAnimation, Widget child) {
+    final finalCurve = curve ?? Get.defaultTransitionCurve;
+    final hasCurve = curve != null;
     if (fullscreenDialog && transition == null) {
+      /// by default, if no curve is defined, use Cupertino transition in the
+      /// default way (no linearTransition)... otherwise take the curve passed.
       return CupertinoFullscreenDialogTransition(
-          primaryRouteAnimation: animation,
+          primaryRouteAnimation: hasCurve
+              ? CurvedAnimation(parent: animation, curve: finalCurve)
+              : animation,
           secondaryRouteAnimation: secondaryAnimation,
           child: child,
-          linearTransition: true);
+          linearTransition: hasCurve);
     }
 
     if (this.customTransition != null) {
       return this.customTransition.buildTransition(
           context,
-          curve,
+          finalCurve,
           alignment,
           animation,
           secondaryAnimation,
@@ -159,6 +167,10 @@ class GetPageRoute<T> extends PageRoute<T> {
               : child);
     }
 
+    /// Apply the curve by default...
+    final iosAnimation = animation;
+    animation = CurvedAnimation(parent: animation, curve: finalCurve);
+
     switch (transition ?? Get.defaultTransition) {
       case Transition.leftToRight:
         return SlideLeftTransition().buildTransitions(
@@ -283,9 +295,9 @@ class GetPageRoute<T> extends PageRoute<T> {
       case Transition.cupertino:
         return CupertinoTransitions().buildTransitions(
             context,
-            curve,
+            hasCurve,
             alignment,
-            animation,
+            hasCurve ? animation : iosAnimation,
             secondaryAnimation,
             popGesture ?? Get.defaultPopGesture
                 ? _CupertinoBackGestureDetector<T>(
@@ -335,39 +347,13 @@ class GetPageRoute<T> extends PageRoute<T> {
                 : child);
 
       case Transition.native:
-        if (GetPlatform.isIOS)
-          return CupertinoTransitions().buildTransitions(
-              context,
-              curve,
-              alignment,
-              animation,
-              secondaryAnimation,
-              popGesture ?? Get.defaultPopGesture
-                  ? _CupertinoBackGestureDetector<T>(
-                      enabledCallback: () => _isPopGestureEnabled<T>(this),
-                      onStartPopGesture: () => _startPopGesture<T>(this),
-                      child: child)
-                  : child);
-
-        return FadeUpwardsPageTransitionsBuilder().buildTransitions(
-            this,
-            context,
-            animation,
-            secondaryAnimation,
-            popGesture ?? Get.defaultPopGesture
-                ? _CupertinoBackGestureDetector<T>(
-                    enabledCallback: () => _isPopGestureEnabled<T>(this),
-                    onStartPopGesture: () => _startPopGesture<T>(this),
-                    child: child)
-                : child);
-
       default:
         if (GetPlatform.isIOS)
           return CupertinoTransitions().buildTransitions(
               context,
-              curve,
+              hasCurve,
               alignment,
-              animation,
+              hasCurve ? animation : iosAnimation,
               secondaryAnimation,
               popGesture ?? Get.defaultPopGesture
                   ? _CupertinoBackGestureDetector<T>(

@@ -188,7 +188,7 @@ class SizeTransitions {
 class CupertinoTransitions {
   Widget buildTransitions(
       BuildContext context,
-      Curve curve,
+      bool useLinearTransition,
       Alignment alignment,
       Animation<double> animation,
       Animation<double> secondaryAnimation,
@@ -196,7 +196,7 @@ class CupertinoTransitions {
     return CupertinoPageTransition(
       primaryRouteAnimation: animation,
       secondaryRouteAnimation: secondaryAnimation,
-      linearTransition: true,
+      linearTransition: useLinearTransition,
       child: child,
     );
   }