Stop timeout should be added to the request timeout

Using the max of the stop timeout and request timeout did not entirely
make sure that a stop timeout greater than a request timeout wouldn't
fail prematurely with a HTTPTimeout exception. The correct behavior is
to add the timeouts together, as the stop timeout is understood to be
part of the "request processing time". Any transport-level timeout thus
comes in addition to that.

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>