title: Loading SpatialPolygonsDataFrame with Leaflet (for R) doesn't work
body: <p>first of all I'm new to R so please bear with me.</p>
  
  <p>What I'm eventually trying to accomplish is to show an interactive map of Amsterdam with Leaflet. For that I'm using RGDAL to read shapefiles.</p>
  
  <p>This <a href="https://files.datapress.com/amsterdam/dataset/gebiedsindeling_amsterdam/2016-03-29T14:33:35/sd2010zw_region.zip" rel="noreferrer">link</a> contains the shapefiles of amsterdam.</p>
  
  <p>I'm using the following code to read the shapefiles and to show the map. </p>
  
  <pre><code>amsterdam &lt;- readOGR(".", layer = "sd2010zw_region", verbose = FALSE)
  
  leaflet(amsterdam) %&gt;%
    addProviderTiles("CartoDB.Positron", options= providerTileOptions(opacity = 0.99)) %&gt;%
      addPolygons(
        stroke = FALSE, fillOpacity = 0.5, smoothFactor = 0.5
      )
  </code></pre>
  
  <p>What I get is the map from CartoDB.Positron but not the 'polygonmap' as the second layer. What I get is a SpatialPolygonsDataFrame containing all sorts of data.</p>
  
  <p>When I use the plot method on the other hand I get the map of Amsterdam</p>
  
  <pre><code>plot(amsterdam, axes=TRUE, border="gray")
  </code></pre>
  
  <p>But I don't want to use plots, I want to use Leaflet :)</p>
  
  <p>What am I doing wrong here?</p>