net.unbound: Merge luaunbound and prosody defaults in absence of user config (fixes #1763) (thanks rgd) add_defaults() is supposed to merge 3 tables, the defaults in luaunbound, the defaults from prosody and any config from the prosody config file. In the case where no `unbound={}` has been in the config, it skips over the merge and returns only the prosody built-in defaults. This results in libunbound skipping reading resolv.conf and uses its default behavior of full recursive resolution. Prior to #1737 there were only two tables, the luaunbound defaults and the prosody config, where bypassing the merge and returning the former did the right thing.