Fix mixed symmetric/asymmetric decryption (#77)

crypto/testdata/message_mixedPasswordPublic

@@ -0,0 +1,32 @@
+-----BEGIN PGP MESSAGE-----
+Version: GopenPGP 2.0.1
+Comment: password = pinata
+
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+gT9xedbgCYAJLKtf7HhaQu1TSQ==
+=W7AQ
+-----END PGP MESSAGE-----