Coverage.
Buff shine on coverage.
Consider 'X-Original-To' header by default in the `to_hostname' filter.

This header is set by Postfix to match the envelope recipient;  including it
allows matching messages for which the domain is BCC'ed or delivered from a
mailing list.

Allow configuration of the headers considred by the 'to_hostname' filter.
Prep 0.23 release.