It seems that adding a restriction to allow_value to raise
CouldNotClearAttribute if nil cannot be set on the attribute in question
broke a lot of people's tests. Really the only reason we added this was
for validate_presence_of -- it rescued CouldNotClearAttribute and
re-raised it as CouldNotSetPasswordError, but only for passwords and
only if has_secure_password was being used. So, I've figured out another
way of performing this check inside of validate_presence_of only to
prevent tests that have nothing to do with from breaking unnecessarily.
The methods failure_message_for_should and failure_message_for_should_not
have been updated to failure_message and failure_message_negated respectively.
Alias to the old methods to remain backwards compatibility with RSpec 2.
Elliot Winkler