Fix email replies

This fixes two bugs:

1. Mailgun recipient variables weren't matching up with the To fields because the recipient keys were email addresses and the To fields included display names (`zach@hackerschool.com` vs. `Zach <zach@hackerschool.com>`). The fix for this (for now) was to go back to just email addresses in the To fields.

2. Replying didn't work depending on your email client because `reply_info` was case-sensitive (base 64 encoded), but email addresses are not. Gmail would keep the case, but Airmail downcased everything, meaning I couldn't reply. The fix for this is to switch to a case-insensitive `ReplyInfoVerifier` that doesn't base 64 encode the serialized data.
I wasn't able to get `assert_nothing_raised` to work how I want, which is for the failing test to _fail_ not _error_. When I tried it, if you rewrite with `assert_nothing_raised` and the test fails, it raises the exception and reports an error instead of a test failure.