Enums in @FormParams should not be enclosed in quotes

Hello,

We encountered an issue with RestyGWT when using enums as FormParams.
RestyGWT encloses enum value in quotes, so payload looks like name=%22value%22 (x-www-form-urlencoded, so quotes turn into %22), instead of expected name=value.
The fix is to add special handling for enum, rather then serializing it as JSON.