Issue 70

This is the fix for issue #70, EquivalentTo isn't compatible with Ignorecase for dictionaries. This fixes CollectionEquivalentContraint and CollectionSubsetConstraint.

I tried making the fix in the CollectionTally class, but it ended up being much more code and harder to read than the equivalent fix in NUnitEqualityComparer. Based on Charlie's comments on a previous attempt at this, I make sure I compare keys with Tolerance.Empty. This will work comparing keys case-insensitive, but that is the same behaviour of the existing DictionariesEqual method in NUnitEqualityComparer. That method actually compares keys with tolerance, so do we want to do that for KeyValuePairs?

All tests pass on TeamCity and I have tested SilverLight.