fix sha2 bug

refs #4 

lua-aws sha2 routine (taken from http://lua-users.org/wiki/SecureHashAlgorithm) seems to have a problem in pre-processing when message length is 55 (mod 64) in bytes. 

sha2 pre-processing is defined as below. (from definition of http://www.wikiwand.com/en/SHA-2 )

```
Pre-processing:
append the bit '1' to the message
append k bits '0', where k is the minimum number >= 0 such that the resulting message
    length (modulo 512 in bits) is 448.
append length of message (without the '1' bit or padding), in bits, as 64-bit big-endian integer
    (this will make the entire post-processed length a multiple of 512 bits)
```

and here is implementation of pre-processing in [above page](http://lua-users.org/wiki/SecureHashAlgorithm)
``` lua
local function preproc (msg, len)
  local extra = 64 - ((len + 1 + 8) % 64)
  len = num2s(8 * len, 8)    -- original len in bits, coded
  msg = msg .. "\128" .. string.rep("\0", extra) .. len
```

if message length is 55 (mod 64), appending the bit '1' to the message ( appending "\128" in the code )
makes total message length 56 (mod 64), so no more zeroes need to be appended. but actually variable 'extra' become 64 in this case. so unnecessary 512 bits '0' appended to message, which causes wrongly calculated sha2 hash value.

for example, following payload

```
POST\nsqs.ap-northeast-1.amazonaws.com\n/\nAWSAccessKeyId=AKIAI3AZWYC2NG6LU2EQ&Action=SendMessage&MessageBody=%7B%22email_id%22%3A2%2C%22contact_id%22%3A%221-xxxxx%40hotmail.es%22%2C%22bulk_id%22%3A1%2C%22email_version_id%22%3A14%2C%22client_id%22%3A1%7D&QueueUrl=http%3A%2F%2Fsqs.ap-northeast-1.amazonaws.com%2F871570535967%2FtestQueue&SignatureMethod=HmacSHA256&SignatureVersion=2&Timestamp=2014-10-16T20%3A21%3A59%2B0900&Version=2012-11-05
```

should be hashed to 

```
570e036eefb1d926c544dcdc2ae37b23a785f1254d1797b5f208cae388369992
```

but actually hashed to

```
4952c897492c10a4b2e8c7cf9f80285ac4b0dfdd8bd6219799e4780be73e2725
```

by old implementation.