Ruby structs do not validate optional members

The ruby library's Thrift::Struct#validate method does not check optional members. This is a vital method because it determines if a message should be sent to a client. Consider a protocol with a struct used as a response message. That struct contains an optional struct foo with optional member a & required member b. If foo is set to a struct without a that is fine. If foo is set to a struct without b validate succeeds and an bad message is sent to a client. The client receives the a Thrift::TransportException instead of a Thrift::ProtocolException,