Improve httpie output

It would be great if the httpie output were to be a bit more 'idomatic'.
I.e. instead of piping json or raw data into the command and manually setting `Accept` and `Content-Type` headers use the built-in support of specifying values (JSON or GET / form) directly as arguments.

No file or test is named. Start by locating the example directive implementation and inspecting how its httpie command output is generated; compare that output with httpie's built-in JSON, GET, and form argument support. Done means the generated examples use those argument forms instead of piped data and manually specified Accept or Content-Type headers.

Written by the indexing model from the issue text.