title: app-route data not becoming null when pattern is not matched
body: <p>Im building this website on Polymer and im having trouble managing going 'back' due to some weird app-route behaviors.</p>
  
  <p>It is a simple website composed of 3 levels of depth and 2 languages (spanish and english). The website shows some architecture projects with their details along with some other sections about the designers, so the route is something like this: </p>
  
  <p>/#/lang/section/ ( for the general navigation ) <br/>
  /#/lang/section/projectId( for when viewing project details )</p>
  
  <p>Im using 1 <code>app-location</code> and 3 <code>app-route</code> in my main app element, like so:</p>
  
  <pre><code>&lt;app-location route="{{route}}" use-hash-as-path&gt;&lt;/app-location&gt;
      &lt;app-route
              route="{{route}}"
              pattern="/:language"
              data="{{languageData}}"
              tail="{{languageSubRoute}}"&gt;&lt;/app-route&gt;
  
      &lt;app-route
              route="{{languageSubRoute}}"
              pattern="/:section"
              data="{{sectionData}}"
              tail="{{sectionSubRoute}}"&gt;&lt;/app-route&gt;
  
      &lt;app-route
              route="{{sectionSubRoute}}"
              pattern="/:projectName"
              data="{{projectData}}"&gt;&lt;/app-route&gt;
  </code></pre>
  
  <p>I pass the project id ( when there is one ) to a child view responsible of showing all projects in a grid, and when one is clicked, there is an href change on the main app-location so the changes are propagated down to the view and the project details are show, like so:</p>
  
  <pre><code>&lt;projects-view name=" [[_getSectionByLanguage(languageData,'Projects')]]"
                                 language="[[languageData.language]]" 
                                 displayed-project="[[projectData.projectName]]"&gt;
  </code></pre>
  
  <p></p>
  
  <p>I can select a project and display its details with no issues, but when i go 'back' on the route ( /#/en/projects for example ) the <code>projectData</code> variable from the third <code>app-route</code> doesnt become null and the, so the project details remain visible. However, if i go back and THEN refresh the page, everything works.</p>
  
  <p>I'm observing the uppermost route variable in my main app element, and printing the sub routes as any changes occur.</p>
  
  <p>Output:</p>
  
  <pre><code>languageSubRoute: {"prefix":"/en","path":"/projects","__queryParams":{}}
  sectionData: {"section":"projects"}
  sectionSubRoute: {"prefix":"/en/projects","path":"","__queryParams":{}}
  projectData: {"projectName":"Some Project"}
  </code></pre>
  
  <p>So as you can see, even when there is no match for the pattern at the third <code>app-route</code>, the data object ( projectData ) doesn't become null</p>
  
  <p>Whats going on ?</p>