Ensure rec_name is a char, text type

Model.default_get with 'default_rec_name' context relies on the fact that
rec_name is a string base field.

issue6630
review36501002

opportunity.py

@@ -435,7 +435,6 @@ class SaleOpportunity(Workflow, ModelSQL, ModelView):
 class SaleOpportunityLine(sequence_ordered(), ModelSQL, ModelView):
     'Sale Opportunity Line'
     __name__ = "sale.opportunity.line"
-    _rec_name = "product"
     _history = True
     _states = {
         'readonly': Eval('opportunity_state').in_(
@@ -510,6 +509,13 @@ class SaleOpportunityLine(sequence_ordered(), ModelSQL, ModelView):
         sale_line.on_change_product()
         return sale_line
 
+    def get_rec_name(self, name):
+        return self.product.rec_name
+
+    @classmethod
+    def search_rec_name(cls, name, clause):
+        return [('product',) + tuple(clause[1:])]
+
 
 class SaleOpportunityReportMixin:
     number = fields.Integer('Number')