Fix bug introduced in 213503cc2d0c23b6
Fix bug introduced in 213503cc2d0c23b6

When introducing the new Uris type, I forgot to make the API endpoint return
the string representations of the type, and not the type itself.

This fixes the sinks endpoint integration tests.

johanneswuerbach/heapster,krzyzacy/heapster,afein/heapster,a-robinson/heapster,shilpamayanna/heapster,fzu-huang/heapster,piosz/heapster,bruceauyeung/heapster,taimir/heapster,apeeyush/heapster,mwielgus/heapster,saiwl/heapster,aegixx/heapster,mdshuai/heapster,thucatebay/heapster,liuhewei/heapster,jfoy/heapster,naxhh/heapster,mwielgus/heapster,bluebreezecf/heapster,piosz/heapster,Huawei-PaaS/heapster,vishh/heapster-1,taimir/heapster,skytap/heapster,luxas/heapster,ravihansa3000/heapster,shilpamayanna/heapster,huangyuqi/heapster,jackzampolin/heapster,huangyuqi/heapster,liubog2008/heapster,ravihansa3000/heapster,krzyzacy/heapster,andyxning/heapster,afein/heapster,wulonghui/heapster,andyxning/heapster,saiwl/heapster,aegixx/heapster,naxhh/heapster,vishh/heapster-1,mwringe/heapster,liuhewei/heapster,Stackdriver/heapster,AlmogBaku/heapster,Huawei-PaaS/heapster,whuwxl/heapster,adfin/statster,thucatebay/heapster,mwringe/heapster,titilambert/heapster,AlmogBaku/heapster,bluebreezecf/heapster,liubog2008/heapster,digideskio/heapster,wulonghui/heapster,vicki-c/heapster,titilambert/heapster,adfin/statster,a-robinson/heapster,bruceauyeung/heapster,skytap/heapster,vicki-c/heapster,digideskio/heapster,luxas/heapster,jackzampolin/heapster,apeeyush/heapster,burmanm/heapster,whuwxl/heapster,Stackdriver/heapster,fzu-huang/heapster,burmanm/heapster,mdshuai/heapster,jfoy/heapster,johanneswuerbach/heapster