title: cartopy - set_extent() extending requested boundary
body: <p>i'm new to cartopy and still learning basic features.</p>
  
  <p>I tried to plot a specific region however, cartopy extended this region and produced a map going up to approximately 85oN when I requested 80oN. Is there a way I can ensure I only get the region I am interested in?</p>
  
  <pre><code>plt.figure(figsize=(5.12985642927, 3))
  ax = plt.axes(projection=ccrs.PlateCarree(central_longitude=-35))
  ax.set_extent([-100, 30, 0, 80])
  ax.coastlines(resolution='110m')
  gl = ax.gridlines(crs=ccrs.PlateCarree(), draw_labels=True,
                linewidth=2, color='gray', alpha=0.5, linestyle='--')
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/KcY4w.png" rel="noreferrer">PC regional map</a></p>