Use http.DefaultTransport.Clone() when configuring proxy

When a proxy is configured, a bare http.Transport was created with only
Proxy and optionally TLSClientConfig set. This Transport was missing all
the defaults that http.DefaultTransport provides: TLSHandshakeTimeout,
IdleConnTimeout, MaxIdleConns, ForceAttemptHTTP2, etc.

Without a proxy, the client implicitly uses http.DefaultTransport with
all these defaults. This created inconsistent behavior where proxy
connections had no idle timeout, no TLS handshake timeout, and no HTTP/2
support.

Clone http.DefaultTransport and overlay proxy settings on top so that
proxy and non-proxy paths have equivalent transport behavior.

Author: telegrapher

ims/client.go

@@ -16,9 +16,8 @@ func (i Config) httpClient() (*http.Client, error) {
 		if err != nil {
 			return nil, fmt.Errorf("proxy provided but its URL is malformed")
 		}
-		t := &http.Transport{
-			Proxy: http.ProxyURL(p),
-		}
+		t := http.DefaultTransport.(*http.Transport).Clone()
+		t.Proxy = http.ProxyURL(p)
 		if i.ProxyIgnoreTLS {
 			t.TLSClientConfig = &tls.Config{InsecureSkipVerify: true}
 		}