minor readability changes
Kohana_EncryptTest_TestableEncrypt to Kohana_EncryptTest_IvStubbed

Renamed Kohana_EncryptTest_TestableEncrypt to
Kohana_EncryptTest_IvStubbed so that I can add
another testable class

Also feed the known IV from the constructor now
instead of setting the property separately
Encryption key B/C compatible padding for PHP 5.6

Added a protected `_normalize_key` method to pad keys with
null bytes similar to PHP versions older than PHP 5.6 this
is called from the constructor when PHP version is above 5.6.0

Added Kohana_EncryptTest_KeyNormalized that always forces key
normalization, used in test_key_normalization.

Added test_key_normalization which tests our null byte padding
for encryption keys only against lower versions of PHP (< 5.6.0)
to see that it matches the internal key padding those PHP
versions already have.