Please append port number to HOST header

[toge]

In RFC 7230(HTTP1.1),

if the connection's incoming TCP port number
differs from the default port for the effective request URI's
scheme, then a colon (":") and the incoming port number (in
decimal form) are appended to the authority component.

While I know your library supports HTTP/1.0,
I'm glad to append port number to HOST header like following:

#-    req.set_header("Host", host_.c_str());
+    req.set_header("Host", (host_ + ":" + std::to_string(port_)).c_str());

[yhirose]

@toge, thanks for the good suggestion. I have implemented it at bd089e9.