Do not call _assertNotContains override in Django newer than 1.6
Do not call _assertNotContains override in Django newer than 1.6

Prior to Django 1.7, assertContains and assertNotContains test helper
methods behaved differently regarding the 'streaming' flag of a
response. This patch fixes the override I made earlier by calling it
conditionally only for the Django version prior to 1.7. This should
prevent problems with response._charset attribute that no longer works
in Django 1.8

Partially Implements: blueprint django18

Change-Id: I10cbc016e51b1ec7b959623989e8c4aec5899cbf

karthik-suresh/horizon,gerrive/horizon,karthik-suresh/horizon,anthonydillon/horizon,philoniare/horizon,BiznetGIO/horizon,doug-fish/horizon,noironetworks/horizon,Mirantis/mos-horizon,karthik-suresh/horizon,vladryk/horizon,endorphinl/horizon-fork,takeshineshiro/horizon,takeshineshiro/horizon,takeshineshiro/horizon,yeming233/horizon,xinwu/horizon,ChameleonCloud/horizon,maestro-hybrid-cloud/horizon,NeCTAR-RC/horizon,idjaw/horizon,watonyweng/horizon,dan1/horizon-x509,anthonydillon/horizon,wolverineav/horizon,Mirantis/mos-horizon,vladryk/horizon,NeCTAR-RC/horizon,bac/horizon,yeming233/horizon,bigswitch/horizon,Tesora/tesora-horizon,coreycb/horizon,bac/horizon,dan1/horizon-x509,maestro-hybrid-cloud/horizon,doug-fish/horizon,NeCTAR-RC/horizon,redhat-openstack/horizon,bigswitch/horizon,sandvine/horizon,vladryk/horizon,NeCTAR-RC/horizon,gerrive/horizon,philoniare/horizon,redhat-cip/horizon,FNST-OpenStack/horizon,FNST-OpenStack/horizon,mdavid/horizon,davidcusatis/horizon,wolverineav/horizon,noironetworks/horizon,Dark-Hacker/horizon,mdavid/horizon,ChameleonCloud/horizon,noironetworks/horizon,mdavid/horizon,saydulk/horizon,takeshineshiro/horizon,Dark-Hacker/horizon,tqtran7/horizon,saydulk/horizon,Tesora/tesora-horizon,saydulk/horizon,endorphinl/horizon,Mirantis/mos-horizon,BiznetGIO/horizon,doug-fish/horizon,watonyweng/horizon,endorphinl/horizon,gerrive/horizon,coreycb/horizon,FNST-OpenStack/horizon,damien-dg/horizon,davidcusatis/horizon,yeming233/horizon,sandvine/horizon,coreycb/horizon,Tesora/tesora-horizon,openstack/horizon,endorphinl/horizon,vladryk/horizon,tqtran7/horizon,endorphinl/horizon-fork,idjaw/horizon,xinwu/horizon,philoniare/horizon,Mirantis/mos-horizon,sandvine/horizon,django-leonardo/horizon,ChameleonCloud/horizon,karthik-suresh/horizon,xinwu/horizon,sandvine/horizon,redhat-openstack/horizon,endorphinl/horizon-fork,bac/horizon,davidcusatis/horizon,philoniare/horizon,django-leonardo/horizon,watonyweng/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,damien-dg/horizon,endorphinl/horizon-fork,openstack/horizon,xinwu/horizon,noironetworks/horizon,redhat-cip/horizon,openstack/horizon,redhat-openstack/horizon,watonyweng/horizon,yeming233/horizon,saydulk/horizon,wolverineav/horizon,Dark-Hacker/horizon,dan1/horizon-x509,redhat-cip/horizon,davidcusatis/horizon,coreycb/horizon,doug-fish/horizon,dan1/horizon-x509,Tesora/tesora-horizon,anthonydillon/horizon,django-leonardo/horizon,endorphinl/horizon,idjaw/horizon,idjaw/horizon,maestro-hybrid-cloud/horizon,openstack/horizon,mdavid/horizon,bac/horizon,wolverineav/horizon,anthonydillon/horizon,bigswitch/horizon,damien-dg/horizon,tqtran7/horizon,tqtran7/horizon,django-leonardo/horizon,redhat-cip/horizon,BiznetGIO/horizon,gerrive/horizon,bigswitch/horizon,FNST-OpenStack/horizon,Dark-Hacker/horizon,maestro-hybrid-cloud/horizon,damien-dg/horizon,redhat-openstack/horizon