net.http: Add request:cancel() method

This is a new API that should be used in preference to http.destroy_request()
when possible, as it ensures the callback is always called (with an error of
course).

APIs that have edge-cases where they don't call callbacks have, from experience,
shown to be difficult to work with and often lead to unintentional leaks when
the callback was expected to free up certain resources.

net/http.lua

@@ -56,6 +56,16 @@ local function destroy_request(request)
 	end
 end
 
+local function cancel_request(request, reason)
+	if request.callback then
+		request.callback(reason or "cancelled", 0, request);
+		request.callback = nil;
+	end
+	if request.conn then
+		destroy_request(request);
+	end
+end
+
 local function request_reader(request, data, err)
 	if not request.parser then
 		local function error_cb(reason)
@@ -105,6 +115,7 @@ function listener.onconnect(conn)
 	end
 	req.reader = request_reader;
 	req.state = "status";
+	req.cancel = cancel_request;
 
 	requests[req.conn] = req;