fix indexing bug: text attribute must return empty string, not None

git-svn-id: svn://svn.cy55.de/Zope3/src/loops/trunk@1347 fd906abe-77d9-0310-91a1-e0d9ade77398

README.txt

@@ -317,8 +317,8 @@ Index attributes adapter
 
   >>> from loops.resource import IndexAttributes
   >>> idx = IndexAttributes(doc1)
-  >>> idx.text() is None
-  True
+  >>> idx.text()
+  ''
 
   >>> idx.title()
   u'doc1 Zope Info'

resource.py

@@ -237,7 +237,7 @@ class IndexAttributes(object):
     def text(self):
         context = self.context
         if not context.contentType.startswith('text'):
-            return None
+            return ''
         data = context.data
         # TODO: transform to plain text
         #return ' '.join((zapi.getName(context), context.title, data)).strip()
@@ -249,7 +249,7 @@ class IndexAttributes(object):
 
 
 class IndexableResource(object):
-    """ Used for TextIndexNG.
+    """ Used for TextIndexNG - obsolete.
     """
 
     implements(IIndexableContent)