Feature: support for 100 Continue responses

If a request sends the  `Expect: 100-continue` header then the server can respond with `HTTP/1.1 100 Continue`.
Currently lua-resty-http will read this as the final response instead of sending the body and receiving the correct response.

https://svn.tools.ietf.org/svn/wg/httpbis/draft-ietf-httpbis/latest/p2-semantics.html#rfc.section.6.2
https://svn.tools.ietf.org/svn/wg/httpbis/draft-ietf-httpbis/latest/p2-semantics.html#header.expect