[FIX] purchase: do not pass uom in product_id_change when changing of product

To allow the on change to set the product default unit of measure when changing of product, and only in this case (not when changing price or quantity)

addons/purchase/purchase_view.xml

@@ -237,7 +237,7 @@
                         <page string="Products">
                             <field name="order_line">
                                 <tree string="Purchase Order Lines" editable="bottom">
-                                    <field name="product_id" on_change="onchange_product_id(parent.pricelist_id,product_id,0,product_uom,parent.partner_id, parent.date_order,parent.fiscal_position,date_planned,name,False,parent.state,context)"/>
+                                    <field name="product_id" on_change="onchange_product_id(parent.pricelist_id,product_id,0,False,parent.partner_id, parent.date_order,parent.fiscal_position,date_planned,name,False,parent.state,context)"/>
                                     <field name="name"/>
                                     <field name="date_planned"/>
                                     <field name="company_id" groups="base.group_multi_company" widget="selection"/>