title: Error: Invalid glob argument for an src path, works as variable
body: <p>Trying to set up gulp to concat some CSS files.</p>
  <p>In my <code>gulpfile.js</code> I have this:</p>
  <pre><code>const paths = {
    css: {
      cssForConcat: [
        './css/*.css',
        '!./css/style.css',
      ],
    }
  }
  </code></pre>
  <p>But this code:</p>
  <pre><code>function styles () {
    return gulp.src([paths.css.cssForConcat])
  </code></pre>
  <p>Returns an error:</p>
  <pre><code>[07:23:27] 'styles' errored after 811 μs
  [07:23:27] Error: Invalid glob argument: ./css/*.css,!./css/style.css
  </code></pre>
  <p>Copying-and-pasting the constant value works OK with no error:</p>
  <pre><code>function styles () {
    return gulp.src('./css/*.css', '!./css/style.css')
  </code></pre>
  <p>What is wrong with my constant definition?</p>