ACK clients on "Expect: 100-continue" header

By responding with a "HTTP/1.1 100 Continue" response to
encourage a client to send the rest of the body.

This is part of the HTTP/1.1 standard but not often implemented
by servers:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.2.3

This will speed up curl uploads since curl sleeps up to 1 second if
no response is received:
http://curl.haxx.se/docs/faq.html#My_HTTP_POST_or_PUT_requests_are

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