title: setContentType("text/html") for JTextPane doesn't work as it is expected
body: <p>When you setContentType("text/html") it is applied only for the text that is set via JTextPane.setText(). All other text, that is put to the JTextPane via styles is "immune" to content type.</p>
  
  <p>Here is what I mean:</p>
  
  <pre><code>private final String[] messages = {"first msg", "second msg &lt;img src=\"file:src/test/2.png\"/&gt; yeah", "&lt;img src=\"file:src/test/2.png\"/&gt; third msg"};
  
  public TestGUI() throws BadLocationException {
      JTextPane textPane = new JTextPane();
      textPane.setEditable(false);
      textPane.setContentType("text/html");
  
      //Read all the messages
      StringBuilder text = new StringBuilder();
      for (String msg : messages) {
          textext.append(msg).append("&lt;br/&gt;");
      }
      textPane.setText(text.toString());
  
      //Add new message
      StyledDocument styleDoc = textPane.getStyledDocument();
      styleDoc.insertString(styleDoc.getLength(), messages[1], null);
  
      JScrollPane scrollPane = new JScrollPane(textPane, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
  
      //add scrollPane to the main window and launch
      //...
  }
  </code></pre>
  
  <p>In general, I have a chat that is represented by JTextPane. I receive messages from server, process them - set text color for specific cases, change smile markers to images path etc. everything is made within the bounds of HTML. But as it can be clearly seen from example above, only the setText is the subject of setContentType("text/html") and the second part, where new message added is represented by "text/plain" (if I'm not mistaken).</p>
  
  <p>Is it possible to apply "text/html" content type to all data that is inserted to JTextPane? Without it, it is almost impossible to process messages without implemention of very complex algorithm.</p>