DELETE an object should not require its dictionary value

[volbap]

Currently, the delete() call in ServerRepository is performing a DELETE to /:entity/:id and passing the JSON representation of the object as a parameter; that last part should not be needed.

Proposed solution:

• Replace self.backend.futureForPath(path, method: .DELETE, parameters: entity.dictionaryValue)
  with self.backend.futureForPath(path, method: .DELETE, parameters: nil)

Reasoning:

• :id is enough for identifying the object that needs to be deleted.