Generate polygon.xml with reference to default_polygon.sld [GEOS-6763]

Ensure polygon.xml is created with a reference to default_polygon.sld

Details: https://jira.codehaus.org/browse/GEOS-6763
Merge pull request #816 from jodygarnett/default_polygon

Generate polygon.xml with reference to default_polygon.sld [GEOS-6763]