A $ref may be a property name and in such cases it's not hashable

This is not a "proper fix" as the proper fix should check if `$ref`
is used as a name of the property, but this works for the purposes of #123

fixes #123

Author: arcivanov

openapi_spec_validator/decorators.py

@@ -18,7 +18,8 @@ def __init__(self, instance_resolver):
 
     def __call__(self, func):
         def wrapped(validator, schema_element, instance, schema):
-            if not isinstance(instance, dict) or '$ref' not in instance:
+            if (not isinstance(instance, dict) or '$ref' not in instance
+                    or not instance['$ref'].__hash__):
                 for res in func(validator, schema_element, instance, schema):
                     yield res
                 return