[tac_plus] Should optional A/V pair be sent?

[heasley]

handling of optional AVs is coded as 'if the nas didnt send it (as
mandatory or optional), then the daemon does not send it'.

the ancient ietf draft does not make this necessary.  its only assertion
is that both sides must ignore optional AVs if they do not support them:

   The arguments in both a REQUEST and a RESPONSE can be specified as
   either mandatory or optional. An optional argument is one that may or
   may not be used, modified or even understood by the recipient.

ISTR someone mentioning on this list that some device of theirs threw a
fit if it received an optional AVP that it didnt understand.  Perhaps
daniel?