Ensure Vonage delivery returns an HTTP::Response

[rbague]

Closes #53 by ensuring the Vonage delivery method returns the HTTP response object in its delivery method, so it can be used in an around callback (as explained here).

I've also added tests for all the delivery methods so that any new changes don't break this "feature". In the end, we'll probably want to standardize the response of the delivery action of all delivery methods (as @excid3 suggested here).

[rbague]

Just for mentioning it, whilst working on this, I realized that the ActionCable and Email delivery methods return "strange" values.
The ActionCable delivery method returns the response of broadcasting though action_cable, which is nil (I don't know if it would make more sense to explicitly return nil, for clarity, and in case the return value of broadcast changes in the future).
And the Email delivery returns the scheduled active_job that will send the email later on (which I don't know if it is useful at all to the user, and it may be better to return nil)

[excid3]

Looking good @rbague 👍

ActionCable - I don't think the return value matters much on that one.
Email - I actually don't know that we need to deliver_later here since we're already in a background job. It's redundant to queue up another job. What if we change it to deliver_now and use that return value?

[rbague]

It does make more sense to deliver the email the same moment the job is executed, instead of triggering a new one. Which also solves the "issue" of returning the mailer job, and now returns a Mail::Message object, which can be useful for debugging.

I've also updated the dummy application so emails are sent

[excid3]

This looks great. Thanks for the help on this @rbague!