http: Verify if we need to get http client support for 100 Continue

[harshavardhana]

golang/go#3665 - waiting on this Go 1.6

[harshavardhana]

8.2.3 Use of the 100 (Continue) Status

The purpose of the 100 (Continue) status (see section 10.1.1) is to allow a client that is sending a request message with a request body to determine if the origin server is willing to accept the request (based on the request headers) before the client sends the request body. In some cases, it might either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking at the body.

Requirements for HTTP/1.1 clients:

  - 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.
  - A client MUST NOT send an Expect request-header field (section
    14.20) with the "100-continue" expectation if it does not intend
    to send a request body.

[harshavardhana]

To keep note of while implementing this feature - https://tools.ietf.org/html/rfc7231#page-35

      A client that receives a 417 (Expectation Failed) status code in
      response to a request containing a 100-continue expectation SHOULD
      repeat that request without a 100-continue expectation, since the
      417 response merely indicates that the response chain does not
      support expectations (e.g., it passes through an HTTP/1.0 server).

Currently being transparently being implemented by http.Client we need to make sure that Golang http.Client would behave this way.