From http://msdn.microsoft.com/en-us/library/office/dn292556.aspx:
To specify multiple parameters, include the parameter as a name-value pair, and separate the parameters with commas. For example:
http://server/site/_api/web/getAvailableWebTemplates(lcid=1033, includeCrossLanguage=true)
So, when trying to activate a feature using the Add method:
POST syntax
POST http://
Resource parameters
- featureId
- force
- featdefScope
What you really need is:
_api/web/features/add(featureId=guid'7845d9e1-238d-4591-a1b3-83e06bd29ee5',force=false,featdefScope=2)