RPC: disable curl's Expect header

This patch solves the very slow (~8-9 seconds) gnt-instance modify
behaviour. Well, it solves in general the slow RPC behaviour, but it was
most visible in that LU.

It seems that curl's behaviour with regard to file uploads (via PUT) and
the 'Expect' header are interacting badly with our http server.

First, our http server doesn't properly handle this header. According to
RFC 2616:

  Requirements for HTTP/1.1 origin servers: Upon receiving a request
  which includes an Expect request-header field with the "100-continue"
  expectation, an origin server MUST either respond with 100 (Continue)
  status and continue to read from the input stream, or respond with a
  final status code.

Our server doesn't do this, and hence it triggers this behaviour in curl
(from the curl FAQ):

  4.16 My HTTP POST or PUT requests are slow!

  libcurl makes all POST and PUT requests (except for POST requests with a
  very tiny request body) use the "Expect: 100-continue" header. This header
  allows the server to deny the operation early so that libcurl can bail out
  already before having to send any data. This is useful in authentication
  cases and others.

  However, many servers don't implement the Expect: stuff properly and if the
  server doesn't respond (positively) within 1 second libcurl will continue
  and send off the data anyway.

  You can disable libcurl's use of the Expect: header the same way you disable
  any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.

This behaviour was detected by watching the captured traffic (in non-SSL
mode), where between the initial HTTP headers (ending with the Expect
one), there was a ~1-2 second pause until curl was sending the body.
Properly RTFM-ing would have saved ~1 day of digging around, but hey…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>