Added an empty content header after the first multipart header.

It seems that after the first 'Content-Type: multipart/mixed;' header another Content-Type: with payload is expected. (http://en.wikipedia.org/wiki/MIME#Multipart_messages)

Fixes #11
This is the server side fix for https://github.com/Qabel/qabel-core/issues/117.
Why do you define another content-type? I thought a body would fix the problem.
By body I mean something like  "Since this is multipart, this text is ignored until the first boundary".