Decide what to do with an “Expect: 100-continue” header.

If the client is expecting a 100 Continue response, we must respond with either a 100 Continue or a final response before waiting for the request body. The default is to always respond with a 100 Continue. You can behave differently (for example, reject unauthorized requests) by overriding this method.

This method should either return True (possibly after sending a 100 Continue response) or send an error response and return False.