[FIX] partner_multi_relation: fix crash on partner with domain [(1, '=', 1)]

As is_leaf test passes with that domain, startswith method cannot be applied
to integers. Converting to string the arg, we avoid the problem.

partner_multi_relation/models/res_partner.py

@@ -143,7 +143,7 @@ class ResPartner(models.Model):
         # pylint: disable=no-value-for-parameter
         date_args = []
         for arg in args:
-            if is_leaf(arg) and arg[0].startswith('search_relation'):
+            if is_leaf(arg) and str(arg[0]).startswith('search_relation'):
                 if arg[0] == 'search_relation_date':
                     date_args = []
                     break
@@ -155,7 +155,7 @@ class ResPartner(models.Model):
         active_args = []
         if self.env.context.get('active_test', True):
             for arg in args:
-                if is_leaf(arg) and arg[0].startswith('search_relation'):
+                if is_leaf(arg) and str(arg[0]).startswith('search_relation'):
                     active_args = [('relation_all_ids.active', '=', True)]
                     break
         return super(ResPartner, self).search(