title: Webpack mix verion files that use Copy or CopyDirectry
body: <ul>
  <li>Laravel Mix Version: 1.4.5</li>
  <li>Node Version: v6.10.2</li>
  <li>NPM Version: 3.10.10</li>
  <li>OS:  windows 10 xampp</li>
  </ul>
  <h3>Description:</h3>
  <p>Undefined index when using copy/copyDirectory and {{ mix() }}</p>
  <h3>Steps To Reproduce:</h3>
  <p>Create 2 folders in resources</p>
  <blockquote>
  <p>/css/afolder/afile.css</p>
  <p>/css/another/anotherfile.css</p>
  </blockquote>
  <p>Copy the folders with the following:</p>
  <pre><code>`mix.copy('resources/assets/css/', 'public/css', false);`
  </code></pre>
  <p><strong>or</strong></p>
  <pre><code>`mix.copyDirectory('resources/assets/css/', 'public/css');`
  </code></pre>
  <p>The files are copied fine, but when using</p>
  <pre><code>`{{ mix('css/afolder/afile.css') }}`
  </code></pre>
  <p><strong>or</strong></p>
  <pre><code>`{{ mix('css/anotherfolder/anotherfile.css') }}`
  </code></pre>
  <p><strong>It returns back</strong></p>
  <blockquote>
  <p>Undefined index: css/afolder/afile.css</p>
  <p>Undefined index: css/anotherfolder/anotherfile.css</p>
  </blockquote>
  <p>The folders exists:</p>
  <pre><code>public/css/afolder/afile.css
  public/css/anotherfolder/anotherfile.css
  </code></pre>
  <p>Mix manifest just shows the following</p>
  <pre><code>{
      &quot;/js/app.js&quot;: &quot;/js/app.js?id=0eaf1649511b8f1c3fd9&quot;,
      &quot;/css/app.css&quot;: &quot;/css/app.css?id=a86d86d0b7edd1152cc6&quot;
  }
  </code></pre>
  <p>Full webpackmix file</p>
  <pre><code>let mix = require('laravel-mix');
  
  /*
   |--------------------------------------------------------------------------
   | Mix Asset Management
   |--------------------------------------------------------------------------
   |
   | Mix provides a clean, fluent API for defining some Webpack build steps
   | for your Laravel application. By default, we are compiling the Sass
   | file for the application as well as bundling up all the JS files.
   |
   */
  
  mix.js('resources/assets/js/app.js', 'public/js');
  
  mix.copy('resources/assets/js/', 'public/js', false);
  
  mix.copy('resources/assets/css/', 'public/css', false);
  
  mix.sass('resources/assets/sass/app.scss', 'public/css');
  
  mix.version();
  </code></pre>