Only send requested-family for IPV6 allocations

[JoTurk]

Description

this helper enforces two constraints on sending REQUESTED-ADDRESS-FAMILY:

1. no RESERVATION-TOKEN (was already enforced before this PR).
2. IPV6 is desired.

should fix #547

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.59%. Comparing base (c9702be) to head (adaa9d6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #549      +/-   ##
==========================================
- Coverage   81.63%   81.59%   -0.04%     
==========================================
  Files          46       46              
  Lines        3191     3195       +4     
==========================================
+ Hits         2605     2607       +2     
- Misses        384      385       +1     
- Partials      202      203       +1     

Flag	Coverage Δ
go	81.59% <100.00%> (-0.04%)	⬇️

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[JoTurk]

@rg0now based on @philipch07 approval and that you voted for this change in the issue, I'm going to merge and tag this to hopefully fix #547 . it shouldn't break anything.

[adrianosela]

Thanks for this @JoTurk