title: Google Cloud Datastore query.add_filter doesn't work correctly
body: <p>I've managed to save some data to Google Cloud Datastore and now I'm planning on querying it to get meaningful data out.</p>
  
  <p>I've tried to follow <a href="https://cloud.google.com/datastore/docs/concepts/queries" rel="nofollow noreferrer">Google Cloud documentation</a> as well as I can, but I cannot get add_filter to work properly with floats.</p>
  
  <p>In this query, I'm trying to find all products that have <code>currentPrice</code> over 500, but instead I get all results just sorted with <code>currentPrice</code>: </p>
  
  <pre><code>from google.cloud import datastore
  
  def create_client(project_id):
      return datastore.Client(project_id)
  
  client = create_client('product-catalog')
  query = client.query(kind='Product')
  
  query.add_filter('currentPrice', '&gt;', 500)
  
  for result in query.fetch():
      for key in result:
          print(key+"("+str(type(result[key]))+"): "+str(result[key]))
      print('---')
  </code></pre>
  
  <p>returns:</p>
  
  <pre><code>title(&lt;class 'str'&gt;): Cheap Oven
  url(&lt;class 'str'&gt;): /product/343
  currentPrice(&lt;class 'float'&gt;): 109.0
  createdAt(&lt;class 'datetime.datetime'&gt;): 2018-03-22 19:52:02.173806+00:00
  ---
  title(&lt;class 'str'&gt;): Regular Oven
  url(&lt;class 'str'&gt;): /product/1231
  currentPrice(&lt;class 'float'&gt;): 549.0
  createdAt(&lt;class 'datetime.datetime'&gt;): 2018-03-21 03:25:24.622558+00:00
  ---
  title(&lt;class 'str'&gt;): Expensive Oven
  url(&lt;class 'str'&gt;): /product/4234
  currentPrice(&lt;class 'float'&gt;): 2399.0
  createdAt(&lt;class 'datetime.datetime'&gt;): 2018-03-23 22:46:01.571207+00:00
  ---
  </code></pre>
  
  <p>I was expecting for my query to result only <code>Regular Oven</code> and <code>Expensive Oven</code>.</p>
  
  <p>For clarity, I've only included three results, but in my actual code there are over 50 000 products and I've verified, that the query results all products sorted by currentPrice and it is not just coincidence.</p>