`request_message_descriptor` should not allow request bodies on `NO_BODY` request types

[inklesspen]

https://github.com/cloudendpoints/endpoints-python/blob/3ccb44c8a0a6835d961e4850e5c51eb92eb92ce5/endpoints/openapi_generator.py#L597-L600

Failure to throw an exception here leads to a confusing exception within body_parameter_descriptor later on, as seen here.