Fix: BePartialViewResult().Model throws NullReferenceException

In ViewResultBaseAssertions.Model, the subject was being cast to
ViewResult.  This caused a NullReferenceException when the subject was a
PartialViewResult.  If the subject is cast to ViewResultBase instead, the
exception is avoided.
Moved partial view result assertion tests to PartialViewResultAssertions_Tests class.
Added Contributors.txt file.
Bumped version to 0.2.1 and updated NuGet release notes.