Add Signing.singer_email (#89)

diff --git a/tests/test_jwt.py b/tests/test_jwt.py
index 2a4795a..3959260 100644
--- a/tests/test_jwt.py
+++ b/tests/test_jwt.py
@@ -264,6 +264,10 @@
         signature = self.credentials.sign_bytes(to_sign)
         assert crypt.verify_signature(to_sign, signature, PUBLIC_CERT_BYTES)
 
+    def test_signer_email(self):
+        assert (self.credentials.signer_email ==
+                SERVICE_ACCOUNT_INFO['client_email'])
+
     def _verify_token(self, token):
         payload = jwt.decode(token, PUBLIC_CERT_BYTES)
         assert payload['iss'] == self.SERVICE_ACCOUNT_EMAIL