Use value instead of record instance on import_data

Importing records with m2o- and m2m-relations import_data gives record instances
to the create-method of the object instead of record-id's. Creating the same
object using the GUI the client gives record-id's.
On error the record-instances can't be serialized by JSON and the common error
"500 internal server error" will be reported to the client

issue3124
review767002

trytond/model/modelstorage.py

@@ -566,7 +566,7 @@ class ModelStorage(Model):
                     warn('too_many_relations_found', value, relation)
                     res = None
                 else:
-                    res = res[0]
+                    res = res[0].id
                 return res
 
             def get_many2many(relation, value):
@@ -586,7 +586,7 @@ class ModelStorage(Model):
                     else:
                         res.extend(res2)
                 if len(res):
-                    res = [('set', res)]
+                    res = [('set', [x.id for x in res])]
                 return res
 
             def get_one2one(relation, value):