fix docs about content_type and serialization
fix docs about content_type and serialization

if serializer is None and content_type is not specified, then serialization will happen (see messaging.Producer:_prepare)
ZoranPavlovic/kombu,ZoranPavlovic/kombu,mverrilli/kombu,daevaorn/kombu,tkanemoto/kombu,tkanemoto/kombu,celery/kombu,Elastica/kombu,jindongh/kombu,jindongh/kombu,numb3r3/kombu,mverrilli/kombu,numb3r3/kombu,Elastica/kombu,urbn/kombu,daevaorn/kombu