title: Updating existing index in pymongo
body: <p>I'm using pymongo and want to update an index from being unique to being unique and sparse.  Both <code>ensure_index</code> and <code>create_index</code> don't seem to want to update the index that already exists.  I'm running mongo 2.2.0</p>
  
  <pre><code>In [1]: coll.index_information()
  Out[1]:
  {u'_id_': {u'key': [(u'_id', 1)], u'v': 1},
   u'foo_1': {u'key': [(u'foo', 1)], u'unique': True, u'v': 1}}
  
  In [2]: coll.ensure_index([('foo', 1)], unique=True, sparse=True)
  
  In [3]: coll.index_information()
  Out[3]:
  {u'_id_': {u'key': [(u'_id', 1)], u'v': 1},
   u'foo_1': {u'key': [(u'foo', 1)], u'unique': True, u'v': 1}}
  
  In [4]: coll.drop_index('foo_1')
  
  In [5]: coll.ensure_index([('foo', 1)], unique=True, sparse=True)
  Out[5]: u'foo_1'
  
  In [6]: coll.index_information()
  Out[6]:
  {u'_id_': {u'key': [(u'_id', 1)], u'v': 1},
   u'foo_1': {u'key': [(u'foo', 1)], u'sparse': True, u'unique': True, u'v': 1}}
  </code></pre>
  
  <p>Any ideas? I can't exactly do what I did here on a production database and just drop the index and re-create it.</p>