Update constant case in all files

@@ -15,7 +15,7 @@ The coordinate system is using the internal Pdf system:
 Example:
 ```dart
 final pdf = new PDFDocument();
-final page = new PDFPage(pdf, pageFormat: PDFPageFormat.LETTER);
+final page = new PDFPage(pdf, pageFormat: PDFPageFormat.letter);
 final g = page.getGraphics();
 final font = new PDFFont(pdf);
 
@@ -24,7 +24,7 @@ g.drawRect(50.0, 30.0, 100.0, 50.0);
 g.fillPath();
 
 g.setColor(new PDFColor(0.3, 0.3, 0.3));
-g.drawString(font, 12.0, "Hello World!", 5.0 * PDFPageFormat.MM, 300.0);
+g.drawString(font, 12.0, "Hello World!", 5.0 * PDFPageFormat.mm, 300.0);
 
 var file = new File('file.pdf');
 file.writeAsBytesSync(pdf.save());

@@ -4,19 +4,19 @@ import 'package:pdf/pdf.dart';
 
 void main() {
   final pdf = new PDFDocument(deflate: zlib.encode);
-  final page = new PDFPage(pdf, pageFormat: PDFPageFormat.LETTER);
+  final page = new PDFPage(pdf, pageFormat: PDFPageFormat.letter);
   final g = page.getGraphics();
   final font = new PDFFont(pdf);
   final top = page.pageFormat.height;
 
   g.setColor(new PDFColor(0.0, 1.0, 1.0));
-  g.drawRect(50.0 * PDFPageFormat.MM, top - 80.0 * PDFPageFormat.MM,
-      100.0 * PDFPageFormat.MM, 50.0 * PDFPageFormat.MM);
+  g.drawRect(50.0 * PDFPageFormat.mm, top - 80.0 * PDFPageFormat.mm,
+      100.0 * PDFPageFormat.mm, 50.0 * PDFPageFormat.mm);
   g.fillPath();
 
   g.setColor(new PDFColor(0.3, 0.3, 0.3));
-  g.drawString(font, 12.0, "Hello World!", 10.0 * PDFPageFormat.MM,
-      top - 10.0 * PDFPageFormat.MM);
+  g.drawString(font, 12.0, "Hello World!", 10.0 * PDFPageFormat.mm,
+      top - 10.0 * PDFPageFormat.mm);
 
   var file = new File('example.pdf');
   file.writeAsBytesSync(pdf.save());

@@ -51,7 +51,7 @@ class PDFPage extends PDFObject {
   PDFPage(PDFDocument pdfDocument, {this.pageFormat})
       : super(pdfDocument, "/Page") {
     pdfDocument.pdfPageList.pages.add(this);
-    if (pageFormat == null) pageFormat = PDFPageFormat.A4;
+    if (pageFormat == null) pageFormat = PDFPageFormat.a4;
   }
 
   /// This returns a PDFGraphics object, which can then be used to render

@@ -6,7 +6,7 @@ import "package:test/test.dart";
 void main() {
   test('Pdf1', () {
     var pdf = new PDFDocument();
-    var page = new PDFPage(pdf, pageFormat: PDFPageFormat.A4);
+    var page = new PDFPage(pdf, pageFormat: PDFPageFormat.a4);
 
     var g = page.getGraphics();
     g.drawLine(30.0, 30.0, 200.0, 200.0);