[Test] Add some test over `yojson` instances

What about basic testing?
For instance, even if we can suppose that yojson is working, having tests like:

when calling to_yojson on this object, it produces the following JSON

or also

when calling to_yojson then from_yojson, it gives back the original object

If we always say "we can do that later" we will never do it... Moreover, some tests are just quick-win and ensure our domain is safe in the real world.

For instance, I think the function http_request isn't so difficult to test, am I wrong?
Or maybe we can start by only testing http_post function, which is exposed and used, and test the others later.
What do you think?

Originally posted by @gsebil08 in https://github.com/marigold-dev/deku/pull/444#discussion_r813671456

Start by locating the yojson conversions and the exposed http_post function, then inspect how http_request is used. Done means tests cover expected to_yojson output and verify that to_yojson followed by from_yojson returns the original object.