Bug 15572. Lookup KnownTypeCollection element types in MSSimpleNamespace

This patch prevents the problem in https://bugzilla.xamarin.com/show_bug.cgi?id=15572. This change allows the DataContractSerializer to deserialize DataMembers that use an array KnownType, at least for the test case in the bug report.
Add tests for bug #15574, pull request 819
Add test for Bug #15572 from pull request #820