Refactor Saml2Binding.Unbind

 - Make Unbind return xml data as a string and let the caller call the correct XXXRequest.Read(xml)
Refactor Saml2Binding.Bind, wrap in new MessageBinder.Bind extension method to get Saml2Binding.Bind and Unbind symetric
Allow xml data to be null
Fix spelling
Refactor: Simplify tests to not test so many layers in each test
Merge pull request #84 from albinsunnanbo/70RefactorBindAndUnbind

70 refactor bind and unbind