Don't lowercasify CAS attribute names

[jalfonso]

This library converts CAS attribute names to lowercase (example).

This is not prescribed by the CAS protocol specification. Note that the CAS protocol documentation even includes an example of a validation response which includes mixed-case attribute names (See Appendix A, "Example response with custom attribute")

This may have implications for the current users of this module. Perhaps a flag could be added to allow case-preserved attributes?

[jalfonso]

Oop, there's even mixed case in the example response in the code comments.

I can work on a pull request if it would help

[nitinhayaran]

bc1609f fixes this