This sends the response, not a request.
Must explicitly remove the Content-Type parameter from smart requests.

Some change in the MIME library changed the need for this apparently.
In Smart requests, the incoming Content-Type only applies to the reply
text, and not the final MIME message we construct server-side. This
used to be automatically dealt with somewhere...

Bug: 13720
BFN
Might as well let Horde_Mime_Mail take care generating the plain part.