defect/sec-sub: Incorrect offset + length used for pkt encryption

Off-by-four bug included the (uninitialized) uwsgi protocol header as part of
the signed packet during encryption, but *not* decryption, effectively preventing successful subscriptions; omit the header from both sides.
Merge pull request #111 from xtfxme/defect/sec-sub

defect/sec-sub: Incorrect offset + length used for pkt encryption