Better check of "in" support

To be sure that "in" is supported
I'd have to check for multiple possible
builtin attributes. Instead I do a try 
except.

src/ZPublisher/BaseRequest.py

@@ -541,9 +541,12 @@ class BaseRequest:
             hasattr(parents[1], 'aq_base') and
             not hasattr(parents[1],'__bobo_traverse__')):
             base = parents[1].aq_base
-            if not (hasattr(base, entry_name) or 
-                (hasattr(base, '__iter__') and entry_name in base)):
-                raise AttributeError(entry_name)
+            if not hasattr(base, entry_name):
+                try:
+                    if not entry_name in base:
+                        raise AttributeError(entry_name)
+                except TypeError:
+                    raise AttributeError(entry_name)
 
         # After traversal post traversal hooks aren't available anymore
         del self._post_traverse