volume: use self.plan.slug for plan param

`class Plan(object)` is not json serializable, the correct action is to
pass it the slug either explicitly or via str(self.plan).

fixes #28

Author: mmlb

packet/Volume.py

@@ -28,7 +28,7 @@ def __init__(self, data, manager):
             self.attached_to = None
 
     def update(self):
-        params = {"description": self.description, "size": self.size, "plan": self.plan, "locked": self.locked}
+        params = {"description": self.description, "size": self.size, "plan": self.plan.slug, "locked": self.locked}
 
         return self.manager.call_api("storage/%s" % self.id, type='PATCH', params=params)