OpenLayer.Filter.Function support for WFS

Hello, Bart asked me to generate a pull request for this based on this thread:
http://osgeo-org.1560.x6.nabble.com/Problems-with-Openlayers-Filter-Function-in-WFS-td5066248.html

The changes fix issues trying to use a Function filter as a Logical filter argument. Also added support for <PropertyName> arguments to Function filters. First timer here, so hopefully I've done this correctly. Comments welcome.