title: How to get order of fields in Scrapy item
body: <p>I'm interested in keeping reference to the order of the field names in a scrapy item. where is this stored?</p>
  
  <pre><code>&gt;&gt;&gt; dir(item)
  Out[7]: 
  ['_MutableMapping__marker',
   '__abstractmethods__',
   '__class__',
   '__contains__',
   '__delattr__',
   '__delitem__',
   '__dict__',
   '__doc__',
   '__eq__',
   '__format__',
   '__getattr__',
   '__getattribute__',
   '__getitem__',
   '__hash__',
   '__init__',
   '__iter__',
   '__len__',
   '__metaclass__',
   '__module__',
   '__ne__',
   '__new__',
   '__reduce__',
   '__reduce_ex__',
   '__repr__',
   '__setattr__',
   '__setitem__',
   '__sizeof__',
   '__slots__',
   '__str__',
   '__subclasshook__',
   '__weakref__',
   '_abc_cache',
   '_abc_negative_cache',
   '_abc_negative_cache_version',
   '_abc_registry',
   '_class',
   '_values',
   'clear',
   'copy',
   'fields',
   'get',
   'items',
   'iteritems',
   'iterkeys',
   'itervalues',
   'keys',
   'pop',
   'popitem',
   'setdefault',
   'update',
   'values']
  </code></pre>
  
  <p>I tried item.keys(), but that returns an unordered dict</p>