Update the URL when changing mailer preview formats

Added javascript to update the URL on mailer previews with the
currently selected email format. Reloading the page now keeps you on
your selected format rather than going back to the default html version.

railties/CHANGELOG.md

+*   Added javascript to update the URL on mailer previews with the currently
+    selected email format. Reloading the page now keeps you on your selected
+    format rather than going back to the default html version.
+
+    *James Kerr*
+
 *   Add fail fast to `bin/rails test`
 
     Adding `--fail-fast` or `-f` when running tests will interrupt the run on

railties/lib/rails/templates/rails/mailers/email.html.erb

@@ -94,7 +94,7 @@
 
     <% if @email.multipart? %>
       <dd>
-        <select onchange="document.getElementsByName('messageBody')[0].src=this.options[this.selectedIndex].value;">
+        <select onchange="formatChanged(this);">
           <option <%= request.format == Mime[:html] ? 'selected' : '' %> value="?part=text%2Fhtml">View as HTML email</option>
           <option <%= request.format == Mime[:text] ? 'selected' : '' %> value="?part=text%2Fplain">View as plain-text email</option>
         </select>
@@ -112,5 +112,19 @@
   </p>
 <% end %>
 
+<script>
+  function formatChanged(form) {
+    var part_name = form.options[form.selectedIndex].value
+    var iframe =document.getElementsByName('messageBody')[0];
+    iframe.contentWindow.location.replace(part_name);
+
+    if (history.replaceState) {
+      var url    = location.pathname.replace(/\.(txt|html)$/, '');
+      var format = /html/.test(part_name) ? '.html' : '.txt';
+      window.history.replaceState({}, '', url + format);
+    }
+  }
+</script>
+
 </body>
 </html>