title: Interpolated array in Grunt template is interpreted as a string
body: <p>Previous title: "Why is Grunt's concat task not using dynamic configuration values?"</p>
  
  <p>I am trying to dynamically configure the files that are concatenated by Grunt, and in doing so I came across this issue where the <code>grunt-contrib-concat</code> plugin does not seem to pick up the dynamically set values. At first I thought I was doing something wrong, but after creating my own task and using the same dynamic values everything came out just as intended. So that leaves the question of why is the grunt concat task not doing picking up and using the same values?</p>
  
  <p>A gruntfile that reproduces the behaviour is seen below (gist: fatso83/73875acd1fa3662ef360).</p>
  
  <pre><code>// Grunt file that shows how dynamic config (and option!) values
  // are not used in the grunt-contrib-concat task. Run using 'grunt'
  module.exports = function(grunt){
  
      grunt.initConfig({
  
          concat : {
              foo : {
                  nonull : true,
                  src: '&lt;%= grunt.config.get("myfiles") %&gt;',
                  dest : 'outfile.txt'
              }
          },
  
          myTask : {
              bar : '&lt;%= grunt.config.get("myfiles") %&gt;'
          }
      });
  
      grunt.loadNpmTasks('grunt-contrib-concat');
      grunt.loadNpmTasks('grunt-contrib-concat');
  
      grunt.registerMultiTask('myTask', function() {
          grunt.log.writeln('myTask:' + this.target + ' data=' + this.data);
      });
  
      grunt.registerTask('default', ['myTask','concat']);
  
      grunt.config.set('myfiles',['file1.txt', 'file2.txt'])
  }
  </code></pre>
  
  <p><strong>EDIT: A new lead:</strong> 
  After literally hours of going nowhere I came across this sentence <a href="http://gruntjs.com/configuring-tasks#files" rel="nofollow">on Grunt's homepage</a>:</p>
  
  <blockquote>
    <p><strong>nonull</strong> If set to true then the operation will include non-matching
    patterns. Combined with grunt's --verbose flag, this option can help
    debug file path issues.</p>
  </blockquote>
  
  <p>Adding that to the config (edited above to reflect this) I got this error message which at least show that something is doing something with the dynamic values:</p>
  
  <pre><code>Running "concat:foo" (concat) task
  &gt;&gt; Source file "file1.txt,file2.txt" not found.
  Warning: Unable to write "outfile.txt/file1.txt,file2.txt" file 
  (Error code: ENOTDIR). Use --force to continue.
  </code></pre>
  
  <p>After some more debugging in the other task, <code>myTask</code>, I have found out that the data sent in to the task as <code>this.data</code> is a string value, not an array. This is perhaps not very surprising, given that we do string interpolation, but this is not consistent with other interpolation features. For instance will <code>&lt;%= otherTask.fooTarget.src %&gt;</code> get the other task's <code>src</code> property as an array value.</p>
  
  <p>Now the question is really how can I avoid passing the interpolated value as an array, rather than a string, to the concat task? </p>