It allows to ignore status of specific job when computed for commit

adding specs for HipChatMessage exposed some issues with the way I was
building the matrix-commit style notification message, so it ended up being
quite a bit more changes than I expected.

The save call from the service configure form submits an empty string as
the server URL if left blank, which I've indicated to do in order to use
the default server, but Hash#merge will happily overwrite a full string with
a blank string if asked, so we need to break that out, along with the worker
options which get mutated into string hash keys, of which the HipChat client
seems to not understand. Additionally, add another spec to make sure we call
the Sidekiq worker with expected arguments.