erp5_web: fix data is not string or buffer on WebPage_exportAsSingleFile

when converting an image, the returned value of img.convert
is not everytime a string. It might be a kind of acquisition
value

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web/WebPage_exportAsSingleFile.py

@@ -172,7 +172,7 @@ def handleImageSourceObject(obj, src):
         format_kw["display"] = value
     if format_kw:
       mime, data = obj.convert(**format_kw)
-      return handleLinkedData(mime, data, src)
+      return handleLinkedData(mime, str(data), src)
 
   return handleHrefObject(obj, src, default_mimetype=bad_image_mime_type, default_data=bad_image_data)