mikel/mail — pull request #587 by defeated (2013-07-24 00:19:01+00:00)
Title: return header fields if Message#headers called without argument

Recently I had to parse specific headers from an incoming email and was "surprised" at the behavior of the `Message#headers` method.

This small change aligns the api of `headers` so that it behaves similarly to the `header` method: if no argument is passed to headers, return the header fields.