Fix return value of Customer#delete_discount

`Customer#delete_discount` has been broken for some time in that it
tries to re-initialize `self` (which is a customer) with a received
discount response. This is incorrect and leads to various problems.

Here, we redefine the return value of `delete_discount` as a discount,
and have it no longer mutate the object on which is was called. We add a
comment as well just to help flag some of the behavior which could
potentially be confusing.

Fixes #963.

lib/stripe/resources/customer.rb

@@ -28,9 +28,14 @@ class << self
       alias detach_source delete_source
     end
 
+    # Deletes a discount associated with the customer.
+    #
+    # Returns the deleted discount. The customer object is not updated,
+    # so you must call `refresh` on it to get a new version with the
+    # discount removed.
     def delete_discount
       resp, opts = execute_resource_request(:delete, resource_url + "/discount")
-      initialize_from(resp.data, opts, true)
+      Util.convert_to_stripe_object(resp.data, opts)
     end
   end
 end