Only Display the Logo in XHTML-capable Text-Filters

Textile and RDoc users can't see the SVG anyway, so don't
bother rendering it.

app/views/layouts/default.rhtml

@@ -39,7 +39,7 @@
 <div id="Container">
 <div id="Content">
   <h1 id="pageName">
-    <%= render(:file => 'svg_logo') %>
+    <%= render(:file => 'svg_logo') if @controller.xhtml_enabled? %>
     <%- if @page and (@page.name == 'HomePage') and %w( show published print ).include?(@action_name) -%>
       <%= h(@web.name) + (@show_diff ? ' (changes)' : '') %>
     <%- elsif @web -%>