Another spelling error fixed.

Author: rohe

tests/test_09_jwt.py

@@ -244,7 +244,7 @@ class DummyMsg(object):
     def __init__(self, **kwargs):
         for key, val in kwargs.items():
             setattr(self, key, val)
-        self.jws_headers = {}
+        self.jws_header = {}
 
     def verify(self, **kwargs):
         return True
@@ -331,4 +331,4 @@ def test_extra_headers():
     bob = JWT(key_jar=_kj, iss=BOB, sign_alg="HS256", typ2msg_cls={"dummy": DummyMsg})
     info = bob.unpack(_jwt)
     assert isinstance(info, DummyMsg)
-    assert set(info.jws_headers.keys()) == {'xtra', 'typ', 'alg', 'kid'}
+    assert set(info.jws_header.keys()) == {'xtra', 'typ', 'alg', 'kid'}