Test that http client requests have expected path, headers, body

[diehuxx]

Currently, we only test that the method -- e.g. TbdexHttpClient.sendMessage or TbdexHttpClient.getOfferings -- does not throw an error when it is called. We should also be testing that the server received a request at the correct path and with the correct headers and body

okhttp has an API to test this with mockWebServer.takeRequest(). See documentation here

[jiyoonie9]

@diehuxx would you say this issue can be closed, or do we need a bit more tests? i see #177 is referenced here, but not sure if that PR included checking all client requests having expected path/headers/body.