title: Adding a GEO2D index to MongoDB via python/pymongo fails repeatedly
body: <p>I tried to drop the "guides" table and rebuild it and ensure index for the "geo" field but it still does not work. Here are the information and errors from pymongo. And also why does ensure index always return "geo_1 " ?</p>
  
  <pre><code>&gt;&gt;&gt; db.guides.ensure_index('geo', pymongo.GEO2D)
  u'geo_1'
  &gt;&gt;&gt; db.guides.index_information()
  {u'geo_1': {u'unique': u'2d', u'key': [(u'geo', 1)], u'v': 0}, u'_id_': {u'key': [(u'_id', 1)], u'v': 0}}
  &gt;&gt;&gt; db.guides.find_one()['geo']
  [61.21572299999999, -149.88431399999999]
  &gt;&gt;&gt; db.guides.find_one({ 'geo' : { '$near' : [62.215, -139.884] }})
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/collection.py", line 469, in find_one
      for result in self.find(spec_or_id, *args, **kwargs).limit(-1):
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/cursor.py", line 601, in next
      if len(self.__data) or self._refresh():
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/cursor.py", line 564, in _refresh
      self.__query_spec(), self.__fields))
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/cursor.py", line 533, in __send_message
      self.__tz_aware)
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/helpers.py", line 100, in _unpack_response
      error_object["$err"])
  pymongo.errors.OperationFailure: database error: can't find special index: 2d for: { geo: { $near: [ 62.215, -139.884 ] } }
  
  &gt;&gt;&gt; db.guides.drop_index('geo_1')
  &gt;&gt;&gt; db.guides.ensure_index('geo', pymongo.GEO2D)
  u'geo_1'
  &gt;&gt;&gt; db.guides.index_information()
  {u'geo_1': {u'unique': u'2d', u'key': [(u'geo', 1)], u'v': 0}, u'_id_': {u'key': [(u'_id', 1)], u'v': 0}}
  &gt;&gt;&gt; db.guides.drop_index('geo_1')
  &gt;&gt;&gt; db.guides.drop_index('geo')
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/collection.py", line 757, in drop_index
      allowable_errors=["ns not found"])
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/database.py", line 293, in command
      msg, allowable_errors)
    File "/usr/local/lib/python2.6/dist-packages/pymongo-1.10-py2.6-linux-x86_64.egg/pymongo/helpers.py", line 126, in _check_command_response
      raise OperationFailure(msg % response["errmsg"])
  pymongo.errors.OperationFailure: command SON([('dropIndexes', u'guides'), ('index', 'geo')]) failed: index not found
  </code></pre>