Bank transfer allows sender and receiver be the same account

[lauripalonen]

In Bank transfer exercise (Securing software, module 2.2.), it is possible for the sender and receiver be the same account. This enables incrementation of balance of a single account, while logically it should stay the same (take amount X from account A, give amount X to account A).

Suggestion: Add a test for checking that transaction is not made if sender == receiver.