title: Use $cond within $match in mongoDB aggregation
body: <p>i've tried to use $cond within $match in one of the stages of an aggregation as shown below :</p>
  
  <pre><code>{ "$match" : { 
    "field1" : { 
                  "$cond" : {
                     "if" : { "$eq" : [ "$id" , 1206]},
                     "then" : 0,
                     "else" : 1545001200
                  }
               }, 
    "field2" : value2 }}
  </code></pre>
  
  <p>But i got this error :</p>
  
  <pre><code>Error:
  Assert: command failed: {
  "ok" : 0,
  "errmsg" : "unknown operator: $cond",
  "code" : 2,
  "codeName" : "BadValue"
  } : aggregate failed
  </code></pre>
  
  <p>The mongodb version is 3.4.4.
  Any idea about this issue?</p>