Updated the unit tests to match the spec in the Readme

It looks like the Readme.md was updated after the unit tests for this problem were written. The unit tests split the ciphertext into word lengths of 5 which contradicts the readme. The readme splits the ciphertext into words that are the height of the square. For example, if the plaintext square is 5 by 4, the word length should be 4.

I modified the existing unit tests and added two new ones with the examples given in the Readme. The update in the example code is the simplest fix.