correctly deals with non serializable type

combine_vars shoudl really be data types, but some just get in
in test, add dict to mock and avoid combine_vars using object

lib/ansible/utils/vars.py

@@ -28,7 +28,8 @@ from ansible.compat.six import iteritems, string_types
 from ansible import constants as C
 from ansible.errors import AnsibleError
 from ansible.parsing.splitter import parse_kv
-from ansible.utils.unicode import to_unicode
+from ansible.utils.unicode import to_unicode, to_str
+
 
 def _validate_mutable_mappings(a, b):
     """
@@ -43,8 +44,14 @@ def _validate_mutable_mappings(a, b):
     # a variable number of arguments instead.
 
     if not (isinstance(a, MutableMapping) and isinstance(b, MutableMapping)):
+        myvars = []
+        for x in [a, b]:
+            try:
+                myvars.append(dumps(x))
+            except:
+                myvars.append(to_str(x))
         raise AnsibleError("failed to combine variables, expected dicts but got a '{0}' and a '{1}': \n{2}\n{3}".format(
-            a.__class__.__name__, b.__class__.__name__, dumps(a), dumps(b))
+            a.__class__.__name__, b.__class__.__name__, myvars[0], myvars[1])
         )
 
 def combine_vars(a, b):

test/units/plugins/strategies/test_strategy_base.py

@@ -193,6 +193,7 @@ class TestStrategyBase(unittest.TestCase):
         mock_inventory.get_host.side_effect = _get_host
         mock_inventory.get_group.side_effect = _get_group
         mock_inventory.clear_pattern_cache.return_value = None
+        mock_inventory.get_host_vars.return_value = {}
 
         mock_var_mgr = MagicMock()
         mock_var_mgr.set_host_variable.return_value = None