title: The `get_map` map function ignores the "source" parameter in ggmap package
body: <p>When using the function <code>get_map()</code> I receive an error prompting to supply an google-API-key though I use the parameter "source":</p>
  
  <pre><code>location = colMeans(city[,c('coords_x1', 'coords_x2')])#mitte
  names(location) &lt;- c('lat', 'lon')
  
  get_map(location = location, source='osm')
  note : locations should be specified in the lon/lat format, not lat/lon.
  Error: Google now requires an API key.
         See ?register_google for details.
  </code></pre>
  
  <p>It seems as if the parameter is ignored.</p>