Update test_wsgiref.py

I'm not adding c0 at add_header as this is already caught by the other PR of seth and if there are other "versions" of inputting headers it should be the same as in seth's PR (and it's only for debug mode)

Author: benediktjohannes

Lib/test/test_wsgiref.py

@@ -854,6 +854,8 @@ def testRaisesControlCharacters(self):
             with self.subTest(c0):
                 base = BaseHandler()
                 headers = Headers()
+                headers["key"] = f"val{c0}"
+                headers.add_header("key", "val")
                 # HTAB (\x09) is allowed in values, but not in names.
                 if c0 == "\t":
                     base["key"] = f"val{c0}"