[FIX] column._domain is now copied instead of being extended in-place.

This commit removes a bug which causes the _domain attribute on the column class to
grow as the fields_get() is repeatedly called.

For instance, displaying the Customer Invoice views (on account.invoice) repeatedly
by switching between the list and form views and causing a full page refresh
(e.g. with ctrl-shift-r) makes the period_id field _column attribute to grow.

Variable aliasing and mutability are so kewl :)

bzr revid: vmt@openerp.com-20120713125728-yhegsthi7jaxi5gc

openerp/osv/orm.py

@@ -1677,7 +1677,7 @@ class BaseModel(object):
                         dom = []
                         if column._domain and not isinstance(column._domain, basestring):
                             dom = column._domain
-                        dom += eval(node.get('domain', '[]'), {'uid': user, 'time': time})
+                        dom = dom + eval(node.get('domain', '[]'), {'uid': user, 'time': time})
                         search_context = dict(context)
                         if column._context and not isinstance(column._context, basestring):
                             search_context.update(column._context)