Don't change iv field when value is the same

[arthurnn]

user = User.new(name: 'hi')
user.name = 'hi'

That code would generate two different ivs.
At first look that is not an issue, but when using attr_encrypted combined
with ActiveRecord this could impose some issues. Like extra writes to the db.
Also, marking some attributes as dirty, when they were actually not changed.

[fixes #216]
also fixes https://stackoverflow.com/questions/47422328/attr-encrypted-updating-all-encrypted-fields-although-i-only-changed-one

[saghaulor]

This duplicates #216. Please see my response as to why this behavior exists here: #216 (comment)

[arthurnn]

I understand the problem and I am investigating a solution. One problem with not changing the ciphertext when you update with the same value is that you prevent the ability to re-encrypt. Re-encryption is useful when you want to do a key rotation, or when changing your current crypto implementation.
I'm exploring how we might still conditionally allow an update to the record when the unencrypted value hasn't changed, so as to allow for re-encryption. However, at this time I do not have a solution that is complete or satisfactory.

(Copying your answer from the other issue)

I think it is sensible the use cases where you want to force an update, for the use cases you described. However, I believe when someone is doing so, they will want to force that behaviour. So the default, in my opinion, should be to not update if the value is the same.

[twanmaus]

When updating a single attribute all encrypted attributes of that class are changed also. This is unwanted default behaviour when using audited recording all changes made by some user. This default behavior could be a configurable option.