curlhttpsink: Only set MIME as content-type if not set by property
Setting the content-type property shall override internally detected MIME types, to make it possible to do as following example (where audio/basic to be used prior to audio/x-mulaw):
gst-launch-1.0 ... ! mulawenc ! audio/x-mulaw,rate=8000,channels=1 ! curlhttpsink location= content-type=audio/basic
Fix will check if content-type exists before overwriting it, but still allowing multipart to change.