Tweak failure message for allow_value

allow_value currently gives an error message that looks like this:

    Expected errors to include "must be greater than 0" when amount is set to 0, got errors: ["amount must be greater than 0.0 (0)"]

This is confusing because the error message here is "must be greater than 0.0",
not "amount must be greater than 0.0".

With this commit the message changes to:

    Expected errors to include "must be greater than 0" when amount is set to 0, got errors: ["must be greater than 0.0 (attribute: "amount", value: 0)"]
(Sorry to close this early btw, didn't know you were looking at it)

No, because `attribute` may be a string or a symbol -- when you add an error to a record via `errors.add`, the first argument can be anything, Rails doesn't implicitly convert it to a string.

So we convert it to a string here, and then we add inspect because I want to show quotes around that string when this message is printed to the console.