Allow registration of TXT key-value pairs with empty keys or values
Allow registration of TXT key-value pairs with empty keys or values

Previous behaviour was to not allow empty keys and to register empty
values as just the key alone (key as opposed to key=).

benoitc/dnssd_erlang,benoitc/dnssd_erlang,amolenaar/nerves_dnssd,PSPDFKit-labs/dnssd_erlang,PSPDFKit-labs/dnssd_erlang