Add test coverage for `jpm post`

The "experimental" `jpm post` command breaks a lot because it has no test coverage. Because we get a bug report after every time it breaks, I think people use it a lot 👯 

Let's add some basic test coverage so that it doesn't break. I think starting with some basic unit tests that mock out all the HTTP action is fine. This would be enough to catch the kind of breakage we have seen.
