Fix HTTP HEAD method

[chenBright]

What problem does this PR solve?

Issue Number: fix #764

Problem Summary:
解析HTTP HEAD请求的回包时，如果回包header中content-length不为0或者transfer-encoding为shunked，on_headers_complete未返回1，则parser会继续解析body。但是HTTP回包是不包含body部分的，就导致会parser一直等到rpc超时。

brpc/src/brpc/details/http_parser.cpp

Lines 1744 to 1752 in 4fa4bcd

	/* Here we call the headers_complete callback. This is somewhat
	* different than other callbacks because if the user returns 1, we
	* will interpret that as saying that this message has no body. This
	* is needed for the annoying case of recieving a response to a HEAD
	* request.
	*
	* We'd like to use CALLBACK_NOTIFY_NOADVANCE() here but we cannot, so
	* we have to simulate it by handling a change in errno below.
	*/

brpc/src/brpc/details/http_parser.cpp

Lines 1753 to 1766 in 4fa4bcd

	if (settings->on_headers_complete) {
	switch (settings->on_headers_complete(parser)) {
	case 0:
	break;
	case 1:
	parser->flags |= F_SKIPBODY;
	break;
	default:
	SET_ERRNO(HPE_CB_headers_complete);
	return p - data; /* Error */
	}
	}

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

---

Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[wwbmmm]

LGTM
另外标题中的DEAD是HEAD误写了，还是有什么特别的含义？

[chenBright]

LGTM 另外标题中的DEAD是HEAD误写了，还是有什么特别的含义？

笔误。标题和commit message都改过来了