Adds methods for retrieving headers from a message.

These methods allow us to retrieve optional headers added to emails.

From what I can tell, we can't delegate retrieval of the headers to [Mail::Message#header](https://github.com/mikel/mail/blob/master/lib/mail/message.rb#L411) and easily access an optional header which has been passed in. All headers are returned or the header value is set via the parameter passed in.

This introduces a discrepancy between how `#header` functions when called on an email in capybara-email and how it functions in `Mail::Message#header`. I'm open to renaming these methods `optional_header` and `optional_headers` to denote that we're retrieving those from the email and to maintain consistency with how the api is currently implemented (delegating to `Mail::Message` when possible).