fix(activemodel): mark return of persisted? and changed_in_place as bool

[G-Rath]

These methods are meant to be overridden, but have default implementations which return false for convenience which has resulted in the generator typing their returns as false rather than bool

Resolves #955

[github-actions]

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activemodel

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[ksss]

@tk0miya
I'm not familiar with type narrowing, but could this change potentially have negative impacts?