RichTextBox: fixed bug that I re-introduced that deals with using custom ITextFormatters.

ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/RichTextBox/RichTextBox.cs

@@ -71,8 +71,12 @@ namespace Microsoft.Windows.Controls
                 return _textFormatter;
             }
             set
+            {
+                if (_textFormatter != value)
                 {
                     _textFormatter = value;
+                    _textFormatter.SetText(Document, Text);
+                }
             }
         }