Add support for HTTP 100 (continue)

[Duncan McGreggor]

From Amazon S3 docs:

To configure your application to send the request headers prior to sending the request body, use 100-continue. For PUT operations, this helps you avoid sending the message body if the message is rejected based on the headers (e.g., authentication failure or redirect).

For more information on 100-continue, go to Section 8.2.3 of http://www.ietf.org/rfc/rfc2616.txt.

[Robert Collins (lifeless)]

Note that 100-continue support in the intarwebs is spotty:
 - few HTTP/1.0 proxies don't do the right thing
 - not all 1.1 proxies do either :(

So we should be sure to dead with degraded situations when we do this.

-Rob