fix bug in pattern formatting

Commas were being misplaced, leading to output like this:

    pattern={InPort=1Vlan=7,,

This patch fixes this and produces out like this:

    pattern={InPort=1,Vlan=7,

Any pattern with more than filter should exercise this bug. For a quick
example, use the katnetic binary in the frenetic repo and run it on a
policy.
Merge pull request #15 from frenetic-lang/format_pattern

Fix bug in pattern formatting