Move pre_request to a more usable place, followup to #382
Move pre_request to a more usable place, followup to #382

Having the hook called so late makes it impossible to overwrite the HTTP request
calling behavior and set self.sent to True in case of possible cache matches
for example.

This way we can interfere before we waste any CPU time to calculate
a request.

psf/requests,revolunet/requests,revolunet/requests,Bluehorn/requests