Don't share http roundtripper between clients

This seems to fix the issues with concurrency.

client.go

@@ -54,7 +54,9 @@ func (o *ClientOpts) withDefaults() *ClientOpts {
 	}
 
 	if o.HTTPClient == nil {
-		o.HTTPClient = new(http.Client)
+		o.HTTPClient = &http.Client{
+			Transport: http.DefaultTransport.(*http.Transport).Clone(),
+		}
 	}
 
 	if o.MaxRedirects == 0 {