title: use lodash to lowercase internal array element
body: <p>I have this object</p>
  
  <pre><code>ob = {
   timePeriod: "Month",
   device: ["A", "B"]
  }
  </code></pre>
  
  <p>when i use </p>
  
  <pre><code>x=_.mapValues(ob, _.method('toLowerCase'))
  </code></pre>
  
  <p><code>x</code> is </p>
  
  <pre><code> timePeriod: "month",
   device: undefined
  </code></pre>
  
  <p>it is not able to lowercase <code>device</code> array.</p>