fix ITEM_PIPELINES setting handling

* item_pipelines variable was unused so the fallback didn't work;
* added support for fallback in case of ITEM_PIPELINIES defined as set.

scrapy/contrib/pipeline/__init__.py

@@ -14,15 +14,14 @@ class ItemPipelineManager(MiddlewareManager):
     @classmethod
     def _get_mwlist_from_settings(cls, settings):
         item_pipelines = settings['ITEM_PIPELINES']
-        if isinstance(item_pipelines, (tuple, list)):
+        if isinstance(item_pipelines, (tuple, list, set, frozenset)):
             from scrapy.exceptions import ScrapyDeprecationWarning
             import warnings
-            warnings.warn('ITEM_PIPELINES defined as a list is deprecated, switch to a dict',
+            warnings.warn('ITEM_PIPELINES defined as a list or a set is deprecated, switch to a dict',
                 category=ScrapyDeprecationWarning, stacklevel=1)
             # convert old ITEM_PIPELINE list to a dict with order 500
             item_pipelines = dict(zip(item_pipelines, range(500, 500+len(item_pipelines))))
-        return build_component_list(settings['ITEM_PIPELINES_BASE'],
-            settings['ITEM_PIPELINES'])
+        return build_component_list(settings['ITEM_PIPELINES_BASE'], item_pipelines)
 
     def _add_middleware(self, pipe):
         super(ItemPipelineManager, self)._add_middleware(pipe)