[2.0] Bringing along the session headers for requests

I noticed that the session configuration headers were not being applied in the requests factory, meaning that anyone who configures the client with a session configuration and then uses the factory to create an old school AFOperation would not get those header values applied to their requests.

The docs have the following comment about `HTTPAdditionalHeaders`, so we should be safe to add them here:

```Note that these headers are added to the request only if not already present.```

A developer could now use `requestWithMethod:...` and `HTTPOperationWithRequest:...` and get all of the proper headers.