title: cartopy problem with pcolormesh 'GeoAxesSubplot' object has no attribute '_hold'
body: <p>Just trying to learn cartopy, but can't even make a simple pcolormesh of wind gust data...</p>
  
  <pre><code>ax = plt.axes(projection = ccrs.LambertConformal())
  cbax = ax.pcolormesh(gust['lon'], gust['lat'], gust['value'], 
                       transform = ccrs.PlateCarree())
  </code></pre>
  
  <p>I get the error <strong>AttributeError: 'GeoAxesSubplot' object has no attribute '_hold'</strong></p>
  
  <p>However, the contfourf plot does work...</p>
  
  <pre><code>cbax = ax.contourf(gust['lon'], gust['lat'], gust['value'], 
                     transform = ccrs.PlateCarree())
  </code></pre>
  
  <p>How do you make pcolormesh figures with cartopy?</p>