Keep the session in tact when forwarding:

Previous commit fucked up the forwarding because the application cleanup
also removed the session data. A forward(), while being another request
from the top, should keep the session in tact.

Instead we only clenaup the request and response, since those will be
rebuilt properly at the start of a new dispatching cycle.

Closes GH #632. Relates to GH #629, GH #633.
Merge branch 'fix/forward-keeps-session'