Siteimprove Plugin does not use system proxy configuration

The siteimprove plugin uses a resteasy client, which does not use the system proxy configuration. Even overwriting info.magnolia.resteasy.client.factory.RestEasyClientFactory and using the HttpClientBuilder to use the systemProperties does not take effects. 
Since we issued that the magnolia author authoring interface is not working behind a proxy at all and we now can not configure the proxy to use, this is a blocking issue for us.
See SIMPROVE-5.