MODE-941 Updated PropFind to filter out custom properties when only certain properties are requested.

The main work for supporting custom properties in PROPFIND was actually done as part of: https://github.com/ModeShape/modeshape/pull/877 (https://issues.jboss.org/browse/MODE-984)

This pull request only solves a bug in that when only certain properties are requested (by name) they were not taken into account in the case of custom properties.