fix reduce computed

[patricklx]

This fixes computed properties that depend on other computed properties.
issue: #4941
Maybe would fix other issues.
Or introduce more... didnt run the tests yet.

From what i saw through debugging is that cacheSet only works if propertyDidChange is not called. It will just be set to undefined when calling propertyDidChange

also fixes: #9313
http://emberjs.jsbin.com/hotilafamu/1/edit?html,css,js,console,output

I've made a gist with the fixed code, it can be included in jsbins to test:
https://rawgit.com/patricklx/641946978a1e3bac833d/raw/6f0d5e84db8a5b04939b30350e211e071a378485/reduced-computed-fix.js

currenly there are some problems related to changeMeta

[wagenet]

See also #9462

[patricklx]

Another issue that would be partially fixed: #3706
http://jsbin.com/zehalemoha/1/edit?html,js,console,output
I'm not sure if this PR would fix the sortedMixin, does it use computed.sort? It doesn't look like it.

But the issue could also be fixed by returning undefined when the value is NaN
http://jsbin.com/xilezoziva/1/edit?js,output

This is caused by Ember.compare returning 0 when comparing any value with NaN

[patricklx]

Checking the test cases:
for example: https://github.com/emberjs/ember.js/blob/master/packages/ember-runtime/tests/computed/reduce_computed_test.js#L614

From what i read here: http://emberjs.com/api/#method_reduceComputed
It is the correct behavior to recompute reduce computed when the callback functions return undefined , which happens in the testcase.

This is now done with this PR, but not expected in the test.
I adapt the testcase, so that addedItem and removedItem return the array.