Fix IPMask marshalling

[fcrisciani]

Fix marshalling and add test

Signed-off-by: Flavio Crisciani flavio.crisciani@docker.com

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2154459). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #1968   +/-   ##
========================================
  Coverage          ?   38.5%           
========================================
  Files             ?     137           
  Lines             ?   27351           
  Branches          ?       0           
========================================
  Hits              ?   10531           
  Misses            ?   15523           
  Partials          ?    1297

Impacted Files	Coverage Δ
drivers/overlay/peerdb.go	8.45% <100%> (ø)

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[mavenugo]

Thanks for taking care of it @fcrisciani . LGTM

[mavenugo]

I will spend some time on this offline to see why the unmarshalling didnt work with IPMask string formating,