bug: api7-lua-resty-http包在解析带有Expect的响应时不符合RFC2616规范

[LiuYawei2019]

Issue description

apisix作为中间代理层，通过ext-plugin-post-resp扩展使用Java插件，代理转发客户端请求到上游服务。其中客户端发起HTTP请求头中包含Expect属性，后端服务在接收到Expect属性时，分别响应HTTP/1.1 100 Continue 回应支持，或者HTTP/1.1 417 Expectation Failed。当Response包含了响应头信息（例如Server属性）后apisix会出现failed to request: couldn't parse HTTP version from response status line: 的错误，客户端收到502的请求响应。

就如上业务场景，参考RFC2616规范内容：

在RFC2616规范中
4.4 Message Length内容如下：

1.Any response message which "MUST NOT" include a message-body (such
     as the 1xx, 204, and 304 responses and any response to a HEAD
     request) is always terminated by the first empty line after the
     header fields, regardless of the entity-header fields present in
     the message.

6 Response内容如下：

After receiving and interpreting a request message, a server responds
   with an HTTP response message.

       Response      = Status-Line               ; Section 6.1
                       *(( general-header        ; Section 4.5
                        | response-header        ; Section 6.2
                        | entity-header ) CRLF)  ; Section 7.1
                       CRLF
                       [ message-body ]          ; Section 7.2

结论：在请求头包含Expect时，响应码100允许返回general-header和response-header信息。

ext-plugin-post-resp 插件实现依赖http.lua代码如下：

local function _handle_continue(sock, body)
    -- _receive_status 方法中会读取1行数据。即：HTTP/1.1 100 Continue
    local status, version, reason, err = _receive_status(sock) --luacheck: no unused
    if not status then
        return nil, nil, err
    end

    -- Only send body if we receive a 100 Continue
    if status == 100 then
        -- 当响应包含Header信息时会被读取到，后续的解析就会因此出错
        local ok, err = sock:receive("*l") -- Read carriage return
        if not ok then
            return nil, nil, err
        end
        _send_body(sock, body)
    end
    return status, version, err
end

建议代码如下：

local function _handle_continue(sock, body)
    local status, version, reason, err = _receive_status(sock) --luacheck: no unused
    if not status then
        return nil, nil, err
    end

    -- Only send body if we receive a 100 Continue
    if status == 100 then
        -- 读取到空行后再做后续处理
        repeat
            local ok, err = sock:receive("*l") -- Read carriage return
            if not ok then
                return nil, nil, err
            end
        until (ok and #ok == 0)
        _send_body(sock, body)
    end
    return status, version, err
end

Environment

• apisix V3.2
• apisix-java-plugin-runner

Minimal test code / Steps to reproduce the issue

1. Postman 通过POST请求接口，Header头中增加Expect: 100-continue属性
2. Tomcat 容器下返回HTTP/1.1 100 Continue
3. Undertow 容器下返回（Undertow官方在2.2.21.Final版本修改并去掉了Content-Length的属性）

HTTP/1.1 100 Continue
Content-Length: 0

What's the actual result? (including assertion message & call stack if applicable)

当出现类似Undertow带有Content-Length的属性响应时，apisix解析报头时会报错

What's the expected result?

正常处理并返回客户端200