title: Rendering Backbone.js view with template variable undefined in the model
body: <p>Is it possible from a backbone.js view to render an underscore.js template with a variable that is not defined in the related Backbone.js model?</p>
  
  <p>take the <code>can't find variable: username</code> error.</p>
  
  <p>the template to be rendered:</p>
  
  <pre><code>&lt;div class="content" id="content"&gt;               
  &lt;div id="form"&gt;
      &lt;form action=""&gt;
          &lt;input id="username" type="text" placeholder="Username" name="username" value= &lt;%=username%&gt; &gt;
          &lt;input id="password" type="password" placeholder="Password" name="password" &gt;
          &lt;ul class="table-view"&gt;
              &lt;li class="table-view-cell"&gt;
                  Remember me
                  &lt;div class="toggle active"&gt;
                      &lt;div class="toggle-handle"&gt;&lt;/div&gt;
                  &lt;/div&gt;
              &lt;/li&gt;
          &lt;/ul&gt;
          &lt;button id="logbtn" class="btn btn-primary btn-block"&gt;Login&lt;/button&gt;
      &lt;/form&gt;    
  
  &lt;button id="renregbtn" class="btn btn-positive btn-block"&gt;Register&lt;/button&gt;
  &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>rendering line from the view:</p>
  
  <pre><code>this.$el.html( this.template_login( this.loginModel ) );
  </code></pre>
  
  <p>line form the loginModel model file:</p>
  
  <pre><code>define([
      'underscore',
      'backbone',
      ],
      function(_, Backbone) {
          var LoginModel = Backbone.Model.extend({
  
              urlRoot: '/login',
              initialize: function(){
                  this.fetch();
              },
          });
  
          return LoginModel;
  });
  </code></pre>
  
  <p>I set the model manually. But still take the can't find variable error as in the first render case.</p>
  
  <p>same model above:</p>
  
  <pre><code>define([
      'underscore',
      'backbone',
      ],
      function(_, Backbone) {
          var LoginModel = Backbone.Model.extend({
  
              urlRoot: '/login',
              defaults: {
                  username: "abc",
              },
              initialize: function(){
                  this.fetch();
              },
          });
  
          return LoginModel;
  });
  </code></pre>
  
  <p>Both cases result in:</p>
  
  <pre><code>ReferenceError: Can't find variable: username
  </code></pre>
  
  <p>Only if I give an hardcoded model json into the template_login it works.</p>
  
  <pre><code>this.$el.html( this.template_login(  {username: "Joe"} ) );
  </code></pre>