fix($httpBackend): fix HTTP PATCH requests in IE8

In Internet Explorer 8, the native XMLHttpRequest object does not allow the
HTTP PATCH method. In order to accomplish a PATCH, you need to use the ActiveX
object. Update the default XHR to use the ActiveX object when doing a PATCH in
IE8. See the following for documentation:

http://blogs.msdn.com/b/ieinternals/archive/2009/07/23/the-ie8-native-xmlhttprequest-object.aspx
http://bugs.jquery.com/ticket/13240