fix: add charset-utf-8 to response header (#104)

Fixes #103

Specify the charset in the header so the browser knows the success message should be treated as UTF-8.

Author: busunkim96

google_auth_oauthlib/flow.py

@@ -517,6 +517,6 @@ def __call__(self, environ, start_response):
         Returns:
             Iterable[bytes]: The response body.
         """
-        start_response("200 OK", [("Content-type", "text/plain")])
+        start_response("200 OK", [("Content-type", "text/plain; charset=utf-8")])
         self.last_request_uri = wsgiref.util.request_uri(environ)
         return [self._success_message.encode("utf-8")]