refactor: replace Dial by DialContext. (#585)

acme/http.go

@@ -26,10 +26,10 @@ var (
 	HTTPClient = http.Client{
 		Transport: &http.Transport{
 			Proxy: http.ProxyFromEnvironment,
-			Dial: (&net.Dialer{
+			DialContext: (&net.Dialer{
 				Timeout:   30 * time.Second,
 				KeepAlive: 30 * time.Second,
-			}).Dial,
+			}).DialContext,
 			TLSHandshakeTimeout:   15 * time.Second,
 			ResponseHeaderTimeout: 15 * time.Second,
 			ExpectContinueTimeout: 1 * time.Second,