guarantee the return value is string.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@18481 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/Document.py

@@ -1066,7 +1066,7 @@ class Document(XMLObject, UrlMixIn, ConversionCacheMixin, SnapshotMixin):
       Converts the content of the document to a textual representation.
     """
     mime, data = self.convert(format='txt')
-    return data
+    return str(data)
 
   security.declareProtected(Permissions.View, 'asEntireHTML')
   def asEntireHTML(self):