title: How to get external boundary using python Shapely library
body: <p>I am trying to get external boundary (Ring) after merging 2 polygons and converting it to GeoJson format. But doesn't seem to be working.</p>
  <p>Below is my code:</p>
  <pre><code>    mergedPolygon = unary_union([poly1, poly2]) # Merging the Polygon
      print(mergedPolygon.exterior) # Gives Linear ring
      # geojson_out = geojson.Feature(geometry=mergedPolygon.exterior, properties={}) # Gives Error
      geojson_out = geojson.Feature(geometry=mergedPolygon.exterior, properties={})  # This works but I need external Ring only.
  </code></pre>