Fix bug in LocalAdapter with an open_email_in_browser_url config

[minhajuddin]

When LocalAdapter is used with an open_email_in_browser_url config, delivering emails throws an error https://github.com/thoughtbot/bamboo/blob/master/lib/bamboo/adapters/local_adapter.ex#L34-L37 returns a {"", 0} which doesn't match https://github.com/thoughtbot/bamboo/blob/master/lib/bamboo/mailer.ex#L206-L209

[germsvel]

Thanks for opening this PR @minhajuddin.

I think this is solving the same thing #589 solved, but I really like the idea of returning the email as the second element in the {:ok, any} response, and also adding the test.

If you feel up for rebasing and introducing those differences, I'd love to get that in. Otherwise, if you feel like #589 solved it well enough, then we can close this PR.

[minhajuddin]

Thanks, @germsvel rebased my PR.

[germsvel]

Thanks! That was fast ⚡

[germsvel]

Thanks for this work!

[SamuelWillis]

Nice, thanks for this improvement!