fix(websteps, webconnectivity): send the correct user agent (#616)

* [forwardport] fix(webconnectivity): send specific user agent (#615)

This forward ports b8c530388e66b2cc86abad26d077202782e4a823 to `master`.

See https://github.com/ooni/probe/issues/1902

* fix(websteps): send the correct user agent

Also related to https://github.com/ooni/probe/issues/1902: let's just
ensure that also websteps behaves in the correct way.

internal/engine/experiment/webconnectivity/control.go

@@ -57,6 +57,7 @@ func Control(
 		BaseURL:    thAddr,
 		HTTPClient: sess.DefaultHTTPClient(),
 		Logger:     sess.Logger(),
+		UserAgent:  sess.UserAgent(),
 	}
 	sess.Logger().Infof("control for %s...", creq.HTTPRequest)
 	// make sure error is wrapped