title: Convert Polygon to MultiPolygon using RGeo gem
body: <p>I have a legacy areas data, where some records are in <code>Polygon</code> and some in <code>MultiPolygon</code> format stored in noSQL DB in JSON format. I need to migrate that data to a PostGIS database with <code>area</code> column, which has type of ST_MultiPolygon.</p>
  
  <p>So, my question is: how can I convert <code>Polygon</code> areas into <code>MultiPolygon</code> using RGeo gem or something similar?</p>
  
  <p>Right now, I'm converting MultiPolygons like that:</p>
  
  <pre><code>multipolygon = RGeo::GeoJSON.decode(location.area)
  PostgisLocation.create(area: multipolygon)
  </code></pre>
  
  <p>But when area is not multipolygon, but polygon, DB throws exception: <code>PG::InternalError: ERROR:  parse error - invalid geometry</code></p>
  
  <p>Example of polygon's text representation:</p>
  
  <pre><code>POLYGON ((-6.00606009808446 54.5901007810674, -6.01003269079491 54.5928954129105, -5.97732358111394 54.5870863982436, -6.00606009808446 54.5901007810674))
  </code></pre>
  
  <p>The stupidest thing that comes to my mind is to replace <code>POLYGON</code> with <code>MULTIPOLYGON</code> word, and add extra <code>()</code> parentheses. But, I believe there should be a better way to convert that using RGeo library. I spent some time reading documentation for RGeo but didn't find anything that could help me.</p>