title: Sorting ngTable doesn't work when heading gets clicked
body: <p>I'm currently working on a table that requires some sorting. Since the project is using Angular (v1.2.12) I started using the <a href="http://bazalt-cms.com/ng-table/" rel="nofollow">ngTable</a> module (v0.3.2).</p>
  
  <p>The default sorting is the title but the year can also be used as sorting option. When I load the page it works fine, but when I click a table heading the clicked column gets sorted but the sorting is not reflected in the header, also the sorting param is no longer set. </p>
  
  <p>When I start debugging I see that the params.sorting() returns: <em>{title: undefined}</em> From that moment it also is no longer possible to click on a sortable header, it just doesn't do anything anymore.</p>
  
  <p>I think I'm missing something, can't seem to find what though</p>
  
  <p>My data is as follows:</p>
  
  <pre><code>[{
      "year": 2014,
      "title": "Golden title"
  }, {
      "year": 2013,
      "title": "Golden title"
  }, {
      "year": 2013,
      "title": "Another title"
  }, {
      "year": 2014,
      "title": "Whetshoverwerd xsade  aas"
  }, {
      "year": 2013,
      "title": "Another brilliant title"
  }, {
  "year": 2013,
      "title": "Wherever I may SOAP"
  }]
  </code></pre>
  
  <p>The view: </p>
  
  <pre><code>&lt;table ng-table="tableParams" class="table"&gt;
      &lt;tbody&gt;
          &lt;tr ng-repeat="document in $data"&gt;
              &lt;td data-title="'Year'" sortable="'year'"&gt;{{document.year}}&lt;/td&gt;
              &lt;td data-title="'Title'" sortable="'title'"&gt;&lt;a href="#"&gt;{{document.title}}&lt;/a&gt;&lt;/td&gt;
          &lt;/tr&gt;
      &lt;/tbody&gt;
  &lt;/table&gt;
  </code></pre>
  
  <p>The view is a directive, </p>
  
  <pre><code>angular.module('appDirectives').directive('myModuleDirective', function () {
      // Runs during compile
      return {
          restrict: 'E',
          templateUrl: 'path/to/view.html',
          replace: true,
          controller: function ($scope, $timeout, $filter, TitleList, ngTableParams) {
  
              $scope.tableParams = new ngTableParams({
                  page: 1, // show first page
                  count: 10, // count per page
                  sorting: {
                      title: 'asc' // initial sorting
                  }
              }, {
                  total: 0, // length of data
                  getData: function ($defer, params) {
                      TitleList.get({}, function (data) {
                          var orderedData = params.sorting() ?
                              $filter('orderBy')(data, params.orderBy()) :
                              data;
  
                          params.total(orderedData.length);
                          orderedData = orderedData.slice((params.page() - 1) * params.count(), params.page() * params.count());
                          $defer.resolve(orderedData);
                      });
                  }
              });
          }
      };
  </code></pre>