fix #457: XAML CC inserts ", " in markup extension property value

src/AddIns/BackendBindings/XamlBinding/XamlBinding/XamlCompletionItemList.cs

@@ -100,7 +100,7 @@ namespace ICSharpCode.XamlBinding
 								int spaces = CountWhiteSpacesAtEnd(context.Editor.GetWordBeforeCaret());
 								int typeNameStart = markup.ExtensionType.IndexOf(':') + 1;
 								
-								if (!(word == "." || word == "," || word == ":") && markup.ExtensionType.Substring(typeNameStart, markup.ExtensionType.Length - typeNameStart) != word) {
+								if (!(word == "." || word == "," || word == ":" || word == "=") && markup.ExtensionType.Substring(typeNameStart, markup.ExtensionType.Length - typeNameStart) != word) {
 									context.Editor.Document.Replace(context.Editor.Caret.Offset - spaces, spaces, ", ");
 									oldOffset += (2 - spaces);
 								}