ENYO-3662, ENYO-3663: fixes for getPath/setPath

Several problems were found in enyo.getPath, enyo.getPath.fast,
enyo.setPath, and enyo.setPath.fast.

The getPath.fast call didn't correctly handle dereferencing computed
properties, so a "getContent" call created for a published property wouldn't
work if you later overrode that property with a computed property.

Also, the complicated branching expressions were shown to be problematic
because they did too much work.  If a getter function returned a falsy value,
it would then look in computed properties or directly on the raw value.
These were re-written to use if trees which don't have the falsy failure case.

Added a unit test to ensure that a falsy computed property returns the right thing,

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Fix enyo.getPath failure for computed props w/ no dependencies

The computed[path] check failed for empty string, which was what
it's value would be for a no-dependency computed property.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #606 from enyojs/getPath-setPath-falsy-fixes

Get path set path falsy fixes

LGTM

Signed-off-by: Cole Davis (cole.davis@lge.com)