Send Content-Type header along with the DistributionPackage's content in forward distribution

Currently SimpleHttpDistributionTransport only adds Connection and Digest http header (if configured to do so) to the http request. When integrating into other systems then sling the API might require the content type of the package transmitted to be present.

I see to options to support that:

a) implement configureable headers for the http transport similar to the timeouts. This might clash with headers set by the implementation
b) let the serializer specify the type of the content it generates. This will be an API change in the core bundle.

From my perspective b) will be simpler to implement.