title: Group duplicate of same value within object of an array
body: <pre><code>[{
      "_id": {
          "year": 2017,
          "month": 4
      },
      "Confirm": 0
  }, {
      "_id": {
          "year": 2017,
          "month": 4
      },
      "Expired": 25
  }, {
      "_id": {
          "year": 2017,
          "month": 4
      },
      "Pending": 390
  }, {
      "_id": {
          "year": 2017,
          "month": 5
      },
      "Pending": 1400
  }]
  </code></pre>
  
  <p>The array above contain same value month and year. Generated from MongoDB Aggregate. And I want to merge them into a single object and preserve whatever keys and values they have. </p>
  
  <p>Expected output:</p>
  
  <pre><code>[{
      month: 4,
      year: 2017,
      Expired: 25,
      Pending: 390
  }, {
      month: 5,
      year: 2017,
      Pending: 1400
  }]
  </code></pre>
  
  <p>I prefer the fastest execution implementation. Underscorejs or native are welcome. Thanks</p>