Allow empty bodies on POST/PUT/PATCH requests in simple_httpclient

Currently, POST/PATCH/PUT requests with simple_httpclient will error out if there is no request body. As far as I know, it is valid to make requests with these verbs and not have a body: http://stackoverflow.com/questions/7323958/are-put-and-post-requests-required-expected-to-have-a-request-body

This removes that requirement, and adds a test that empty bodies are allowed.