ParallelChannel似乎无法设置子controller的http header

[bzs1118]

大概是为了考虑使用其他协议时的效率，动态创建subdone的controller时并未拷贝用户的controller指定的http header相关参数，因此将默认发出http get。目前的版本是否存在解决方案？

[chenBright]

可以参考UT

brpc/test/brpc_channel_unittest.cpp

Lines 2453 to 2458 in c32ddee

	brpc::ParallelChannel channel;
	for (size_t i = 0; i < NCHANS; ++i) {
	brpc::Channel* sub_chan = new brpc::Channel();
	SetUpChannel(sub_chan, true, false, NULL, "", false, brpc::PROTOCOL_HTTP);
	ASSERT_EQ(0, channel.AddChannel(sub_chan, brpc::OWNS_CHANNEL, new EchoHttpHeader, NULL));
	}

brpc/test/brpc_channel_unittest.cpp

Lines 2429 to 2443 in c32ddee

	class EchoHttpHeader : public brpc::CallMapper {
	public:
	brpc::SubCall Map(int channel_index, int channel_count,
	const google::protobuf::MethodDescriptor* method,
	const google::protobuf::Message* request,
	google::protobuf::Message* response) override {
	return brpc::SubCall(method, request, response->New(), brpc::DELETE_RESPONSE);
	}
	void MapController(int channel_index, int,
	const brpc::Controller* main_cntl,
	brpc::Controller* sub_cntl) override {
	sub_cntl->http_request().SetHeader(ECHO_HTTP_HEADER, std::to_string(channel_index));
	}
	};