souphttpclientsink: If no proxy is set by properties, use the default libsoup proxy resolver

That is, use whatever system settings there might exist. This is the same
behaviour we use in the HTTP source.

ext/soup/gstsouphttpclientsink.c

@@ -583,6 +583,7 @@ gst_soup_http_client_sink_start (GstBaseSink * sink)
           soup_session_async_new_with_options (SOUP_SESSION_ASYNC_CONTEXT,
           souphttpsink->context, SOUP_SESSION_USER_AGENT,
           souphttpsink->user_agent, SOUP_SESSION_TIMEOUT, souphttpsink->timeout,
+          SOUP_SESSION_ADD_FEATURE_BY_TYPE, SOUP_TYPE_PROXY_RESOLVER_DEFAULT,
           NULL);
     } else {
       souphttpsink->session =