Fix duplicate content type

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

brpc/src/brpc/details/http_message.cpp

Lines 576 to 583 in 2729272

	if (!h->content_type().empty()) {
	os << "Content-Type: " << h->content_type()
	<< BRPC_CRLF;
	}
	for (HttpHeader::HeaderIterator it = h->HeaderBegin();
	it != h->HeaderEnd(); ++it) {
	os << it->first << ": " << it->second << BRPC_CRLF;
	}

如果一次http请求中，同时调用了set_content_type和SetHeader("Content-Type")，那么MakeRawHttpRequest后的http包中会有两个Content-Type header。

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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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