title: Choropleth in folium: can not find key_on
body: <p>I have boundaries in geo-json format</p>
  
  <pre><code>{'type': 'FeatureCollection',
   'features': [{'type': 'Feature',
     'properties': {'STATEFP10': '04',
      'ZCTA5CE10': '85347'},
     'geometry': {'type': 'Polygon',
      'coordinates': [[[-113.587371, 32.77836],
        [-113.587269, 32.77819],
        [-113.590707, 32.776887],
       ...
  </code></pre>
  
  <p>I have also some data</p>
  
  <pre><code>data
  
  POSTCODE    X
  0   85347   0
  1   85356   0
  </code></pre>
  
  <p>I am trying to plot a basic Choropleth.
  If I do the following, I get</p>
  
  <pre><code>AttributeError: 'NoneType' object has no attribute 'get'
  </code></pre>
  
  <p>What is wrong in the following snippet? It is an example copied and pasted...
  I have already ensured boundaries keys and dataframe postcodes to be exactly same set...</p>
  
  <pre><code>m = folium.Map( zoom_start=8)
  # Add the color for the chloropleth:
  folium.Choropleth(
   geo_data=boundaries,
   name='choropleth',
   data=data,
   columns=['POSTCODE','X'],
   key_on='features.properties.ZCTA5CE10',
   fill_color='BuPu',
  #fill_opacity=0.7
  ).add_to(m)
  folium.LayerControl().add_to(m)
  m
  </code></pre>