http: don't accept "Expect: 100-CONTINUE"

Because RFC 2616 says the following at "8.2.3 Use of the
100 (Continue) Status":

> - If a client will wait for a 100 (Continue) response before
>   sending the request body, it MUST send an Expect request-header
>   field (section 14.20) with the "100-continue" expectation.