[FIX] base_partner_merge: cleaner test for function fields

After commit 0ed63d73a6,
the hack used to detect fields.function is not supported
anymore. Using `isinstance` is safer and cleaner anyway
(performance is not a concern here).

addons/crm/base_partner_merge.py

@@ -245,12 +245,12 @@ class MergePartnerAutomatic(osv.TransientModel):
         for record in proxy.browse(cr, openerp.SUPERUSER_ID, record_ids, context=context):
             try:
                 proxy_model = self.pool[record.model]
-                field_type = proxy_model._columns[record.name].__class__._type
+                column = proxy_model._columns[record.name]
             except KeyError:
                 # unknown model or field => skip
                 continue
 
-            if field_type == 'function':
+            if isinstance(column, fields.function):
                 continue
 
             for partner in src_partners: