Should Node.enabledProperty respect pickable:null?

[zepumph]

Over while reviewing changes in #1100 (comment), @samreid and I were curious if it is buggy that enabled can change the pickability from null to true silently and not explicitely. Here is the case we came up with in that comment:

x = new Node();
  x.pickable => null;
  x.enabled => true;
x.enabled = true; // no op
  x.pickable => null; // null right now
  x.enabled => true;
x.enabled = false;
  x.pickable => false;
  x.enabled => false;
x.enabled = true;
  x.pickable => true; // buggy? Is this a discrepancy in behavior? Or perhaps a feature?
  x.enabled => true;

@jonathanolson, can you speak to this at all. Do you foresee any problems coming from toggling into true instead of respecting null?

[jonathanolson]

I generally believe enabledProperty should not change the pickability, AND we wouldn't want to replace things with pickable:true generally.

[zepumph]

This is going to be covered by phetsims/sun#666