Send 100-continue only when handler reads body

Currently the default expect100 handler just sends 100-continue immediately.
We could improve this greatly and provide useful Expect functionality by default if we only sent the 100-Continue when the handler tries to read the body. This would mean, for example, that a redirect based on headers could be sent without the client needing to send the body (auth checks etc. would be a good example).

Currently, the code lives in web_urldispatcher.py. This will probably not be an easy task, as we may need to move some logic to the request object.