Adds support for message/coffeepot

According to the [errata][1] the correct Content-Type should be
`message/coffeepot`. However, legacy implementations of the HTCPCP have
used `application/coffee-pot-command` instead.

This patch adds the support for both Content-Types. Fixes issue #1

[1]: http://www.rfc-editor.org/errata_search.php?rfc=2324