THRIFT-223 Validate method should check that enum types are assigned valid values

THRIFT-223 Validate method should check that enum types are assigned valid values

Enum classes now get a VALID_VALUES Set that contains all of the acceptable values. Structs' validate() method has been extended to compare an enum field's value to the valid value set, but only if the __isset flag for that field is true, meaning that non-beans generated code will not perform this validity check unless the __isset flag is manually maintained.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@735910 13f79535-47bb-0310-9956-ffa450edef68