Change incorrect literal check (#138)

Author: nikhilym

ask-sdk-core/ask_sdk_core/api_client.py

@@ -146,6 +146,6 @@ def _convert_dict_to_list_tuples(self, headers_dict):
             for key, values in six.iteritems(headers_dict):
                 for value in values.split(","):
                     value = value.strip()
-                    if value is not None and value is not '':
+                    if value is not None and value != '':
                         headers_list.append((key, value.strip()))
         return headers_list