Add creation date to response cache.
Add creation date to response cache.

Haven't needed this yet, but seems like it'll be useful to have around.
Could probably use file modification date in a pinch, but something
explicit seems safer.

proversity-org/edx-app-ios,appsembler/edx-app-ios,knehez/edx-app-ios,adoosii/edx-app-ios,adoosii/edx-app-ios,proversity-org/edx-app-ios,proversity-org/edx-app-ios,adoosii/edx-app-ios,nagyistoce/edx-app-ios,chinlam91/edx-app-ios,edx/edx-app-ios,edx/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,chinlam91/edx-app-ios,keyeMyria/edx-app-ios,yrchen/edx-app-ios,adoosii/edx-app-ios,lovehhf/edx-app-ios,proversity-org/edx-app-ios,keyeMyria/edx-app-ios,lovehhf/edx-app-ios,appsembler/edx-app-ios,appsembler/edx-app-ios,Ben21hao/edx-app-ios-new,nagyistoce/edx-app-ios,knehez/edx-app-ios,edx/edx-app-ios,knehez/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,Ben21hao/edx-app-ios-new,appsembler/edx-app-ios,appsembler/edx-app-ios,proversity-org/edx-app-ios,knehez/edx-app-ios,edx/edx-app-ios,keyeMyria/edx-app-ios,keyeMyria/edx-app-ios,nagyistoce/edx-app-ios,edx/edx-app-ios,yrchen/edx-app-ios,Ben21hao/edx-app-ios-new,edx/edx-app-ios,proversity-org/edx-app-ios,yrchen/edx-app-ios,lovehhf/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,Ben21hao/edx-app-ios-enterprise-new,Ben21hao/edx-app-ios-enterprise-new,edx/edx-app-ios,Ben21hao/edx-app-ios-new,chinlam91/edx-app-ios,chinlam91/edx-app-ios,Ben21hao/edx-app-ios-new,knehez/edx-app-ios,nagyistoce/edx-app-ios,lovehhf/edx-app-ios,nagyistoce/edx-app-ios