No support for key-only TXT strings

[fzs]

I noticed that the library will ignore txt entries that have only a key but no value, i.e. are of the form txt key. Is there a reason other than it not being implemented?
It looks like the implementation was done according to RFC 1464 and not the mDNS-SD RFC 6763. While the former specifies to ignore TXT strings that have no unquoted = in them, the latter explicitly allows key-only entries as boolean flags.