title: lodash add keys to countBy function
body: <p>I have a datasource as follows</p>
  
  <pre><code>[
    {
      "physicalId": 2110,
      "closedDate": "2017-06-25T00:00:00.000Z",
      "allDay": true
    },
    {
      "physicalId": 2111,
      "closedDate": "2017-06-24T00:00:00.000Z",
      "allDay": true
    },
    {
      "physicalId": 2111,
      "closedDate": "2017-06-25T00:00:00.000Z",
      "allDay": true
    },
    {
      "physicalId": 4299,
      "closedDate": "2017-06-24T00:00:00.000Z",
      "allDay": true
    },
    {
      "physicalId": 4299,
      "closedDate": "2017-06-25T00:00:00.000Z",
      "allDay": true
    }
  ]
  </code></pre>
  
  <p>and I'm using <code>lodash v4.17.4</code> to summarize the data in the following way:</p>
  
  <pre><code>[
    {
      "Sat Jun 24 2017 00:00:00 GMT+0000": 2,
      "Sun Jun 25 2017 00:00:00 GMT+0000": 3,
    }
  ]
  </code></pre>
  
  <p>However, i would like this returned as object in the format of </p>
  
  <pre><code>[
    {
      date: "Sat Jun 24 2017 00:00:00 GMT+0000"
      total: 2
    },
      {
      date: "Sun Jun 25 2017 00:00:00 GMT+0000"
      total: 3
    }
  ]
  </code></pre>
  
  <p>I've not been able to find a way to complete this in lodash. How can I achieve this?</p>