fix string/buffer sjcl issue

...by using sjcl.mode.cbc.encrypt/decrypt rather than sjcl.encrypt/decrypt. The
difference is that the sjcl.encrypt/decrypt functions are really convenience
methods designed to encrypt and decrypt strings, but don't play nice with
binary data, as revealed in the tests in this commit and the previous commit.
Basically, if you use them to encrypt and decrypt binary data as a string, it
will return the wrong result or an error.

The solution is to use the block cipher directly, in this case sjcl.mode.cbc.
This also has the advantage of fewer format conversions - no converting to
base64 and JSON strings. This makes things faster. Also, it is actually correct
unlike the previous method.