Revert "Do not duplicate http.DefaultTransport"

This reverts commit d79f8563.

Kamil pointed out that shallow-copying http.DefaultTransport
accidentally gives us a references to a lot of things that probably
not be shared.

internal/badgateway/roundtripper.go

@@ -17,6 +17,12 @@ var DefaultDialer = &net.Dialer{
 	KeepAlive: 30 * time.Second,
 }
 
+var DefaultTransport = &http.Transport{
+	Proxy:               http.ProxyFromEnvironment, // from http.DefaultTransport
+	Dial:                DefaultDialer.Dial,        // from http.DefaultTransport
+	TLSHandshakeTimeout: 10 * time.Second,          // from http.DefaultTransport
+}
+
 type RoundTripper struct {
 	Socket                    string
 	ProxyHeadersTimeout       time.Duration
@@ -61,8 +67,7 @@ func (t *RoundTripper) configureRoundTripper() {
 		return
 	}
 
-	// Clone http.DefaultTransport. Needs a cast from http.RoundTripper to *http.Transport.
-	tr := *(http.DefaultTransport.(*http.Transport))
+	tr := *DefaultTransport
 	tr.ResponseHeaderTimeout = t.ProxyHeadersTimeout
 
 	if t.Socket != "" {