IsImmutable does not throw an error

[wisepotato]

I would expect isImmutable property of Attr to throw an error when i am updating an entity

tests:

• If i update a todo with immutable property (x) and mutable property (y), i expect an error (401/403 whatever) to be thrown or atleast for me to define the expected behaviour
• If i update a todo with immutable property (x) only I expect an error

follow up: Can i define a property to not be settable on create?

[maurei]

Fixed in #558