title: Using $lte and $gte with find command does not yield any error or result
body: <p>My collection name: <strong>stu</strong></p>
  
  <pre><code>{ "_id" : ObjectId("56a1f7a6f432505f3a6b287b"), "name" : "user1", "mark1" : "100", "mark2" : "200" }
  { "_id" : ObjectId("56a1fd3cf432505f3a6b287c"), "name" : "user2", "mark1" : "89", "mark2" : "70" }
  </code></pre>
  
  <p>Query I wrote </p>
  
  <pre><code>&gt; db.stu.find({"mark1":{$gt:50}}).pretty()
  </code></pre>
  
  <p>What is the problem here? <code>gt,lt gte,lte</code> are also not working.</p>