ENYO-2301: fix XHR code to only apply cache-control code for iOS 6, not 6+. Clarify comments to note this behavior.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #309 from enyojs/ENYO-2301-unwiredben

ENYO-2301: fix XHR code to only apply cache-control code for iOS 6, not ...