Encrypted attribute spec handles non-exception-raising case

* Decrypting with the wrong encryption key is not guaranteed
to raise a CipherError. In the case where the decrypt succeeds,
we check that the decrypted value does not match the original plaintext.

* We intend this to fix the flakiness of this test, especially in Travis runs.

Signed-off-by: Chris Adams <cadams@pivotallabs.com>