`allow_value` will now raise a CouldNotSetAttribute error if the
attribute in question cannot be changed from a non-nil value to a nil
value, or vice versa. In other words, these are the exact cases in which
the error will occur:
* If you're testing whether the attribute allows `nil`, but the
attribute detects and ignores nil. (For instance, you have a model
that `has_secure_password`. This will add a #password= method to your
model that is defined in a such a way that you cannot clear the
password by setting it to nil -- nothing happens.)
* If you're testing whether the attribute allows a non-nil value, but
the attribute fails to set that value. (For instance, you have an
ActiveRecord model. If ActiveRecord cannot typecast the value in the
context of the column, then it will do nothing, and the attribute will be
effectively set to nil.)
What's the reasoning behind this change? Simply put, if you are assuming
that the attribute is changing but in fact it is not, then the test
you're writing isn't the test that actually gets run. We feel that this
is dishonest and produces an invalid test.
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