Fix swift2thrift service generation

This fixes a bug that swift2thrift won't work if you don't explicitly provide a non-UNSPECIFIED requiredness on all @ThriftField annotations on all parameters in your service. Though the @ThriftField annotation default for 'requiredness' is UNSPECIFIED, for a method parameter, this should resolve to NONE.
(cherry picked from commit e5032dac3b4e24c8dead9b0792b57dda1299b40e)