Add public property google.auth.credentials.Signing.signer (#110)

diff --git a/tests/test_jwt.py b/tests/test_jwt.py
index 3959260..e4a9a0a 100644
--- a/tests/test_jwt.py
+++ b/tests/test_jwt.py
@@ -264,6 +264,9 @@
         signature = self.credentials.sign_bytes(to_sign)
         assert crypt.verify_signature(to_sign, signature, PUBLIC_CERT_BYTES)
 
+    def test_signer(self):
+        assert isinstance(self.credentials.signer, crypt.Signer)
+
     def test_signer_email(self):
         assert (self.credentials.signer_email ==
                 SERVICE_ACCOUNT_INFO['client_email'])