Fix align-content:strech overriding align-item

Summary:
Fix for #413. This was a hangover from a previous attept to fix other align-content problems.
Closes https://github.com/facebook/yoga/pull/417

Reviewed By: astreet

Differential Revision: D4604727

Pulled By: emilsjolander

fbshipit-source-id: 92fd31a385d8182c6b201c891d5ae478372d525d

gentest/fixtures/YGAlignContentTest.html

@@ -142,3 +142,9 @@
   <div style="height: 50px;"></div>
   <div style="height: 50px;"></div>
 </div>
+
+<div id="align_content_stretch_is_not_overriding_align_items" style="align-content:stretch;">
+  <div style="width:100px; height: 100px; align-items: center; flex-direction: row; align-content:stretch;">
+    <div style="height: 10px; width: 10px; align-content:stretch;"></div>
+  </div>
+</div>