Fixes for ENYO-3563, ENYO-3562

Related issues fixed in this single PR. Updated unit-tests and tested by hand against 35+ samples to verify working as intended. While there is __no public api changes__ there was a change to the way mixins are applied in that properties specified will always be used - replacing a hack-fix that was filtering them out but this was not the intended use (e.g. being able to reset a default property value to false was impossible). It means that collision needs to be avoided when not explicitly resetting the value such as entries that were defined with the value `null` which are now marked in documentation alone.