Fix a bug where the request failed when upstream sends multiple HTTP/1.1 100 Continue messages

[puiterwijk]

Please note that this IS valid according to RFC2616, section 10.1: A client MUST be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a 100 (Continue) status message. Unexpected 1xx status responses MAY be ignored by a user agent.