A constant true or false property which ignores all method calls.
As an alternative, this method creates a BooleanProp which is true if the key exists in the map.
A BooleanProp with a liberal truth policy
The java definition of property truth is that the key be in the map and the value be equal to the String "true", case insensitively.
A BooleanProp which acts like java's Boolean.getBoolean