Http Retriever should save headers and promote content-type

Currently, the Http Retriever in Link3 provides Link with almost no information about the messages it fetches. The only value it adds to the context is a filename, and it creates no additional files.

I think it would be useful to at least have the option for the retriever to write the content-type of the response to the context, to give the Initialize process better information on how to disassemble the message.

It would also be useful to be able to optionally save the headers and bodies of the request and response the same way AS2 does it. If there is a problem with an endpoint Link provides virtually no information to help with diagnosing the issue.