Made NameValueCollectionAdapter.Contains is now case-insensitive

Pretty straight forward fix to #37

There doesn't appear to be a way to get a IEqualityComparer out of a NameValueCollection post-initialization, so I went with case-insensitive. It would be pretty easy to have an overload(or optional parameter) to accept an IEqualityComparer, but that didn't seem immediately necessary.

I'm relatively new to contributing to OSS so if I missed anything, please let me know.