Use default client in diagnose command

defaultHTTPClient may behave differently to http.DefaultClient and is used for almost all widgets, using it within the diagnose
command will make debugging and replicating issues easier

internal/glance/diagnose.go

@@ -165,7 +165,7 @@ func testHttpRequestWithHeaders(method, url string, headers map[string]string, e
 		request.Header.Add(key, value)
 	}
 
-	response, err := http.DefaultClient.Do(request)
+	response, err := defaultHTTPClient.Do(request)
 	if err != nil {
 		return "", err
 	}