Added XmlElement test and possible fixes to fix test

Had a problem with a generated XmlAny property from a xsd. 
I added the XmlElement to a test and made a few fixes to allow the test to succeed.

One of the changes is to support ICollection like IEnumerable, I also added a test for this.
I added one extra Assert to the IEnumerable test to detect the return error I made previously.

All Unit Test succeeded except 2 unrelated
 - test_rect_different_cultures("de") failed with System.NotSupportedException
 - test_rect_different_cultures("en") failed with System.NotSupportedException