title: MongoDB query returns "Error: Invalid JSON object" on valid query
body: <h2>Issue</h2>
  
  <p>I have probably mucked up the syntax for the JSON query, but cannot for the life of me see where. Trying to create data.frame overviewData grouped on four values with an extra COUNT-feature. </p>
  
  <h2>Code</h2>
  
  <pre><code>overviewData &lt;- M_CONNECTION$aggregate('[
          { 
              "$group" : {
                  "_id" : {
                      "Hotel_Name" : "$Hotel_Name", 
                      "lat" : "$lat", 
                      "lng" : "$lng", 
                      "Average_Score" : "$Average_Score"
                  }, 
                  "COUNT(Hotel_Name)" : {
                      "$sum" : NumberInt(1)
                  }
              }
          }, 
          { 
              "$project" : {
                  "Hotel_Name" : "$_id.Hotel_Name", 
                  "lat" : "$_id.lat", 
                  "lng" : "$_id.lng", 
                  "Average_Score" : "$_id.Average_Score", 
                  "COUNT(Hotel_Name)" : "$COUNT(Hotel_Name)", 
                  "_id" : NumberInt(0)
              }
          }
      ]',
  options = '{"allowDiskUse" : true}'
  )
  </code></pre>
  
  <p>I have quotes around all of my queries, as well as around the options. Still getting an "Invalid JSON object"-error message.</p>