title: Inserting None values into DynamoDB using Boto
body: <p>I'm new to Boto, and I'm trying to use it to insert a Python dictionary into Amazon DynamoDB.  I must be missing something, because the "dynamizer" (encoder) does not seem to support None values.  This is a problem because the source data has tons of nulls in it.  I could go through each row and delete out all of the key/value items where the value is None, but somehow I feel like a package as sophisticated as Boto should take care of that for me.  I'm simply trying to insert one row like so:</p>
  
  <pre><code>conn = DynamoDBConnection(region=RegionInfo(endpoint="dynamodb.us-west-2.amazonaws.com"))
  dest = Table('d_company', connection=conn)
  data = {"company_id":99999, "company_name":None}
  dest.put_item(data)
  </code></pre>
  
  <p>...and this gives me the error: </p>
  
  <pre><code>Error
  Traceback (most recent call last):
    File "TestDynamoDB.py", line 37, in testPutIntoDynamoDB
      dest.put_item(data)
    File "C:\Python27\lib\site-packages\boto\dynamodb2\table.py", line 452, in put_item
      return item.save(overwrite=overwrite)
    File "C:\Python27\lib\site-packages\boto\dynamodb2\items.py", line 362, in save
      final_data = self.prepare_full()
    File "C:\Python27\lib\site-packages\boto\dynamodb2\items.py", line 265, in prepare_full
      final_data[key] = self._dynamizer.encode(value)
    File "C:\Python27\lib\site-packages\boto\dynamodb\types.py", line 228, in encode
      dynamodb_type = self._get_dynamodb_type(attr)
    File "C:\Python27\lib\site-packages\boto\dynamodb\types.py", line 220, in _get_dynamodb_type
      return get_dynamodb_type(attr)
    File "C:\Python27\lib\site-packages\boto\dynamodb\types.py", line 110, in get_dynamodb_type
      raise TypeError(msg)
  TypeError: Unsupported type "&lt;type 'NoneType'&gt;" for value "None"
  </code></pre>
  
  <p>What am I doing wrong?</p>