FIX postgres can filter on non text fields

This issue causes the comparison to fail if comparing a non text fields (for example, a date or integer). This will cause the CMS to fail in places such as Gridfield selector. 

Similar fix as this one: https://github.com/silverstripe/silverstripe-framework/pull/2242 The difference being comparisonClause(...) being more apt solution in SS 3.1
Merge pull request #26 from torleif/patch-1

FIX postgres can filter on non text fields