title: Folium choropleth key_on - I'm missing something obvious
body: <p>I'm trying to make a choropleth of installed PV capacity by postcode. 
  I've got the installation data in a pandas Series (tas_kW_series), index is postcode as int.
  I've downloaded the ESRI shape files for Australia, converted them to GeoJson (using an online tool). Visually the format is correct. I had to split the Geojson into states to keep the size down, the individual jsons work with a geojson layer, but not a choropleth</p>
  
  <p>I can get the shapes to display on a Geojson layer, but the choropleth is failing, on the 'key_on' parameter I think, and I just can't figure out what's wrong with it - it maps to the correct part of the geojson, and the input data is a series so no need to provide column names.</p>
  
  <p>I've had to add a new 'properties' field ('pc_int') with the postcode converted to 'int' to match the index of the series. This is in the json file</p>
  
  <p>The data had more postcodes than the geojson, I've removed the excess</p>
  
  <p>I've checked every way I know how that the structure of the geojson matches the 'feature.properties.pc_int' I'm using in the caller.</p>
  
  <p>I've stepped through the source code, and I'm pretty sure it's correct</p>
  
  <pre><code>Postcode Data (following tas_kW_series.to_dict())
  {7000: 2027.2620000000002,
   7001: 94.36,
   7002: 13.5,
   7004: 910.943,
   7005: 1644.2499999999998,
   7006: 25.1,
   7007: 612.258,
   7008: 1525.3100000000004,
   7009: 1478.017,
   7010: 1876.863,
   7011: 1770.325,
   7012: 205.06,
   7015: 1557.3830000000003,
   7016: 233.579,...
  </code></pre>
  
  <p>GeoJson (with some editing to remove polygon details)
  TAS_data = json.load(TAS.json)</p>
  
  <pre><code>TAS_data['features'][0] = 
  
  {'type': 'Feature',
   'geometry': {'type': 'Polygon',
    'coordinates': [[[147.3199543290001, -42.85551239399996],
      [147.32003433700004, -42.85554737699994],
      [147.32014234000007, -42.85558237899994],
      [147.3202043540001, -42.855599380999934],...
  ...
      [147.31961900400006, -42.856079991999934],
      [147.31967809700006, -42.85600952499993],
      [147.3199543290001, -42.85551239399996]]]},
   'properties': {'POA_CODE16': '7000',
    'POA_NAME16': '7000',
    'AREASQKM16': 10.8544,
    'pc_int': 7000}}
  </code></pre>
  
  <p>Choropleth call - </p>
  
  <pre class="lang-py prettyprint-override"><code>folium.Choropleth(geo_data = 'TAS.json', 
                    data = tas_kW_series,
                    name = 'Tasmania STC Installations - Total Capacity (kW)',
                    key_on = 'feature.properties.pc_int',
                    bins = 10,
                    fill_opacity = 0.7,
                    line_opacity = 0.2,
                    legend_name = 'Installed Capacity (kW)').add_to(m)
  </code></pre>
  
  <p>traceback</p>
  
  <pre><code>---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-46-7bcc2e3b37d6&gt; in &lt;module&gt;
        7                   fill_opacity = 0.7,
        8                   line_opacity = 0.2,
  ----&gt; 9                   legend_name = 'Installed Capacity (kW)').add_to(m)
  
  ~\AppData\Local\conda\conda\envs\mlbook\lib\site-packages\folium\features.py in __init__(self, geo_data, data, columns, key_on, bins, fill_color, nan_fill_color, fill_opacity, nan_fill_opacity, line_color, line_weight, line_opacity, name, legend_name, overlay, control, show, topojson, smooth_factor, highlight, **kwargs)
      932         self._name = 'Choropleth'
      933 
  --&gt; 934         if data is not None and not color_brewer(fill_color):
      935             raise ValueError('Please pass a valid color brewer code to '
      936                              'fill_local. See docstring for valid codes.')
  
  ~\AppData\Local\conda\conda\envs\mlbook\lib\site-packages\branca\utilities.py in color_brewer(color_code, n)
      147 
      148     if base_code not in core_schemes:
  --&gt; 149         raise ValueError(base_code + ' is not a valid ColorBrewer code')
      150 
      151     try:
  
  ValueError: blue is not a valid ColorBrewer code
  </code></pre>
  
  <p>expected output is a choropleth layer on a folium map, shaded according to binning of values in tas_kW_series. actual output is traceback above.</p>