Fix deleting labels from an article (#528)

Following a81e575, reading and creating labels for articles work as expected - but deleting a label ends up removing the label from the instance _entirely_ (which removes it from all articles)

This is due to the `destroy!` action preferring the `url` v. `path` (which, in the case of article labels, `url` returns the [delete label](https://developer.zendesk.com/api-reference/help_center/help-center-api/article_labels/#delete-label) endpoint, while `path` returns the [delete label from article endpoint](https://developer.zendesk.com/api-reference/help_center/help-center-api/article_labels/#delete-label-from-article).

Author: codealchemy

lib/zendesk_api/resources.rb

@@ -232,6 +232,12 @@ class Label < DataResource
       include Read
       include Create
       include Destroy
+
+      def destroy!
+        super do |req|
+          req.path = path
+        end
+      end
     end
     has_many Label
   end