Adding 100-continue in WSGI response headers

See http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.2.3 for background on the http client header Expect: 100-continue.

The purpose of this client header and its associated response is to allow a client to determine if a request body will be accepted before sending it. This would be useful in cases of bulk image uploads, where several problems may take place, such as authentication failures, authorization failures, a missing image, etc. With this header a client submitting an unacceptable request to upload an image would find out about the failure before writing the entire image to the http socket.