THRIFT-553. rb: thrift structs should be comparable (<=>)

THRIFT-553. rb: thrift structs should be comparable (<=>)
This patch adds the spaceship operator to the struct and union base classes, enabling object comparisons between objects without regenerating code.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@911644 13f79535-47bb-0310-9956-ffa450edef68