clean up secret key docs

consistent key across docs and examples
consistent key across tests, set in conftest

tests/conftest.py

@@ -18,12 +18,7 @@ from flask import Flask as _Flask
 
 class Flask(_Flask):
     testing = True
-    secret_key = __name__
-
-    def make_response(self, rv):
-        if rv is None:
-            rv = ''
-        return super(Flask, self).make_response(rv)
+    secret_key = 'test key'
 
 
 @pytest.fixture