Update UrlEncodedFormat.php

In the case where parameters from types int, float or string (maybe other too) are passed in the url standard query string with value '0' (for instance ?v1=0), Restler returns an error (Bad Request: `v1` is required but missing).
Merge pull request #220 from ogodon/patch-3

Update UrlEncodedFormat.php to accept zero as a value