fix(reports) do not send unnecessary attributes in 'ping' message
fix(reports) do not send unnecessary attributes in 'ping' message

Prior to this patch, configuring a plugin via `POST /plugins` would send
reports with `<entity>=userdata;<entity>=userdata;` when the plugin was
not configured on a given entity (e.g. when configuring a global plugin,
or a plugin with a Route only).

We also avoid sending `enabled=true` which has very little value and
takes space in our already limited UDP packet length.

From #4156
Kong/kong,Kong/kong,Kong/kong,Mashape/kong