Enqueue offline

[eric1234]

dba18c9 is really the goal of my PR. I want offline mails to be enqueued just like ActionMailer. My personal need is for the normal instrumentation events to kick off but it also has the following advantages:

• Better parity with ActionMailer.
• Ability for calling code to decide if it should immediately queue or queue later.

The other commits are entirely just some cleanup to make the test run cleanly. Quick summary:

• 5ed0580 - Allows tests to pass under Rails 5 where the default adapter is async now. Test were assuming inline (old default)
• faeab9c - Removal of rspec deprecation warning.
• 180a2ca - Removal of logging noise when tests run

Each commit message has further info if desired.

[renz45]

Hey thanks for this. I apologize in taking so long to get back to you, I was out of town for most of last week. This looks good to me 👍 I'll cut a new release as well.