Added handling for "Expect: 100-continue" header to
BaseHTTPRequestHandler. By default, any request that has this header
gets a 100 Continue response (with no other headers) before
do_WHATEVER() is called. By overriding handle_expect_100(), you can
reject incoming requests instead of sending a 100 Continue if you so desire.

Refactoring as per comments above was also performed.

Note: This patch changes the default behaviour in the case where both
the client and the server claim to support HTTP/1.1 from doing nothing
in the case of an "Expect: 100-continue" header on the request to
sending a 100 Continue response and then completing the request as normal.