response description: in str format

hapic/doc.py

         schema_class = type(description.output_body.wrapper.processor.schema)
         method_operations.setdefault('responses', {})\
             [int(description.output_body.wrapper.default_http_code)] = {
-                'description': int(description.output_body.wrapper.default_http_code),  # nopep8
+                'description': str(int(description.output_body.wrapper.default_http_code)),  # nopep8
                 'schema': {
                     '$ref': '#/definitions/{}'.format(schema_class.__name__)
                 }
         )
         method_operations.setdefault('responses', {})\
             [int(description.output_file.wrapper.default_http_code)] = {
-            'description': int(description.output_file.wrapper.default_http_code),  # nopep8
+            'description': str(int(description.output_file.wrapper.default_http_code)),  # nopep8
         }
 
     if description.errors:
             schema_class = type(error.wrapper.error_builder)
             method_operations.setdefault('responses', {})\
                 [int(error.wrapper.http_code)] = {
-                    'description': int(error.wrapper.http_code),
+                    'description': str(int(error.wrapper.http_code)),
                     'schema': {
                         '$ref': '#/definitions/{}'.format(schema_class.__name__)  # nopep8
                     }

tests/func/test_doc.py

         assert 'responses' in doc['paths']['/upload']['post']
         assert 500 in doc['paths']['/upload']['post']['responses']
         assert {
-            'description': 500,
+            'description': "500",
             'schema': {
                 '$ref': '#/definitions/DefaultErrorBuilder'
             }