Apply the RFC7516 standard to the Response structure

[Xynnn007]

We had found that KBS Protocol does not follow JWE standard quite well, and we had done some work to fix this up.

#44
#45

The two patches add support to AEAD's aad field to JWE body, and also a format for ProtectedHeader.

When I rechecked the standard, I found that there are still some pieces we do not follow quite well.

1. The ProtectedHeader must be a base64 encoding string of the struct.
2. When serializing the Response in Flattened JWE JSON Serialization Syntax, fields encrypted_key, aad, iv, ciphertext, tag must be BASE64URL_NOPAD encoded.

I am sorry that the previous commits from my side did not fix the things perfectly. Thus I put this issue to catch some attention if anything is missed.