title: Query DynamoDB ISO-8601 Date index with less than condition
body: <p>I have a DynamoDB Index storing a date in iso-8601 format, [i.e. '2017-03-17']. I'd like to query the Index for dates less than a certain date. [i.e datetime.now() - 30 days].  </p>
  
  <p>I believe this is support from looking at Amazon's API docs. I seem to be missing something with the boto3 syntax.</p>
  
  <p>I am using the resource and not the client in boto3.</p>
  
  <p>I know I can use epoch but would like to use the ISO format for readability.</p>
  
  <p>Here is the test code I have.</p>
  
  <pre><code>import boto3
  from boto3.dynamodb.conditions import Key, Attr
  
  ddb = boto3.resource('dynamodb')
  _table = ddb.Table('TableWithDateIndex')
  
  response2 = _table.query(
      IndexName='DeleteDate-index',
      KeyConditionExpression=Key('IndexDate').lt('2017-04-17')
  )
  
  print response2['Items']
  </code></pre>
  
  <p>Fails with:</p>
  
  <p>botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the Query operation: Query key condition not supported</p>