Do not leave <img> tag with an empty src field!!

fixed #189

lib/tables.py

@@ -11,7 +11,7 @@ output (using standard tags like <table> and co.).
 import inspect
 import lxml
 from django.utils.translation import ugettext as _
-from django.template import Template, Context
+from django.template import Template, RequestContext, Context
 from django.template.loader import render_to_string
 
 class Column:
@@ -193,7 +193,7 @@ class Table(object):
   {% include "common/table_body.html" %}
 </table>
 """)
-            return t.render(Context({
+            return t.render(RequestContext(self.request, {
                         "table" : self, "tableid" : self.tableid
                         }))
         t = Template("""

templates/common/table_head.html

@@ -7,7 +7,7 @@
     <th id="{{ c.name }}" width="{{ c.width }}">
     {% endif %}
       {{ c|safe }}
-      <span style="float: right"><img style="display: none" alt="" src="" /></span>
+      <span style="float: right"><img style="display: none" alt="" src="{{ MEDIA_URL }}pics/transparent.png" /></span>
     </th>
     {% endfor %}
   </tr>