U1 must be released this month ! I'm also waiting for it 
For now you can try with xml parameter if you want to try ?
And if you just want to run a workflow, why are you posting to https://192.168.1.7:8281/api/workflows/dec0e608-8b83-4e32-8825-3e77ef31c4d7/presentation/instances and not to this URL : https://192.168.1.7:8281/api/workflows/dec0e608-8b83-4e32-8825-3e77ef31c4d7/executions/
? As it is explain in the documentation : http://pubs.vmware.com/vsphere-51/index.jsp?topic=/com.vmware.vsphere.vco_develop_web_services.doc/GUID-5312E7E5-90B7-4D14-9BDA-43BF22E0DDF4.html