Correct evaluation for "Acct-Interim-Interval" from RADIUS

Setting "Acct-Interim-Interval :=600" in FreeRadius2 evaluates to 'random' values with PfSense 2.1.
Possibly a bug related to:
https://forum.pfsense.org/index.php?topic=60079.0
https://forum.pfsense.org/index.php?topic=60262.0
Merge pull request ##1285 from dariomas/patch-1