[FIX] base/ir_qweb: use _fields instead of _all_columns

This fixes cases with computed fields that are not present in _all_columns.

openerp/addons/base/ir/ir_qweb.py

@@ -582,7 +582,7 @@ class FieldConverter(osv.AbstractModel):
             ('data-oe-type', field_type),
             ('data-oe-expression', t_att['field']),
         ]
-        if record._all_columns[field_name].column.readonly:
+        if record._fields[field_name].readonly:
             data.append(('data-oe-readonly', 1))
         return data