fail to deal with request header Expect: 100-Continue and respone

hi @pintsized ,

Bug 1

the following point is case sensitive,so if the value of request header Expect has some upper character,we will make mistake without handle_continue;

• http.lua#L539
• http.lua#L557
• http.lua#L641

Bug 2

_handle_continue only try to read an empty line,but there is some oter optional headers like Date or Content-length. see rfc2616-10.1 Informational 1xx.

Bug 3

http.lua#L557 only handle continue response when Expect: 100-continue request header is there.but as the RFC 2616 say:

- An origin server SHOULD NOT send a 100 (Continue) response if
the request message does not include an Expect request-header
field with the "100-continue" expectation, and MUST NOT send a
100 (Continue) response if such a request comes from an HTTP/1.0
(or earlier) client.

see RFC2616#sec8.2.3.

It's SHOULD NOT requirement,it would be nice if we support it.

I have made a fix and tested it ok.

thanks.

Start in lib/resty/http.lua at the referenced lines around _handle_continue and the Expect header checks. Compare handling of case-insensitive Expect values, optional headers in 100 responses, and unsolicited 100 responses with the cited RFC sections. Done means all three reported cases are handled correctly and the existing tests or reproductions pass.