lmj010308 fixed /http/server/HttpResponseWriter.cpp 使用SSEvent时,当event 为空指针时，不加event参数

[lmj010308]

int HttpResponseWriter::SSEvent(const std::string& data, const char* event /* = "message" */) {
if (state == SEND_BEGIN) {
EndHeaders("Content-Type", "text/event-stream");
}
std::string msg="";
if(event != nullptr){
msg = "event: "; msg += event; msg += "\n";
}
msg += "data: "; msg += data; msg += "\n\n";
state = SEND_BODY;
return write(msg);
}