title: Drupal8 REST-api for taxonomy terms do not work
body: <p>I am writing an Angular2 web app with Drupal8 in the backend. I need to convert taxonomy id's to their respective labels, for that I want to use the API endpoint "/taxonomy/term/{taxonomy_term}". I have enabled the correct REST-API and given access to all roles, however when I try to access that end-point with an ID and ?_format=json, I get the message '{"message":"Not acceptable format: json"}'. If I remove the ?format=json, however, it gives me the results I want in HTML.</p>
  
  <p>This error message is found in the file /core/lib/Drupal/Core/EventSubscriber/AcceptNegotiation406.php and is preceded by the following comment:</p>
  
  <pre><code>   // If this is a render array then we assume that the router went with the
      // generic controller and not one with a format. If the format requested is
      // not HTML though we can also assume that the requested format is invalid
      // so we provide a 406 response.
  </code></pre>
  
  <p>Can anyone help me with this? Alternatively, I will have to scrape the taxonomy label from the HTML-result.</p>