In case of SOURCE we are lenient and thus quite some source clients
don't send a proper content-type, especially if they only support mp3.

This was meant to be introduced in 2.4.0 already, sadly we missed it.

All source clients MUST send proper content-type after migrating to 
Icecast HTTP PUT protocol.

closes #2082

svn path=/icecast/trunk/icecast/; revision=19288