The common procedure for HTTP Basic Auth is that a client does /not/
immediately send out credentials via an 'Authorization'-header, but to
wait until the server tells the client to do so - which the server
indicates via the 'WWW-Authenticate'-header.
PowerDNS-Admin (and flask in general), though, abort the whole
communication if no Authorization header was found in the initial
request - resulting in '200 "badauth"'.
While this might work for /some/ HTTP clients - which right away add an
Authorization header crafted from provided credentials (via args or
extracted from given URL), this is /not/ standard and /not/ common.
Hence add the 'WWW-Authenticate'-header for every unauthenticated call
checking for dyndns authorisation.
Note, though, this changes the status code from 200 to 401 in this case,
which - given the explanation why 200 was chosen in the first place -
might cause side effects.
