I know there are HTTP mocking/stubbing frameworks, but at least when I researched last time they didn't allow me to validate requests efficiently and they were just for HTTP, not for example for raw TCP exchanges, this system allows me to have a unified framework for all API communication.
Assuming you just want to quick&dirty convert a hash to json, send the json to a remote host to test an API and parse response to ruby this is probably fastest way without involving additional gems: