Fix AM/AR Dirty plugin issues with AR 5.2

[shioyama]

Fixes #115

[shioyama]

@pwim So I found a way to fix the problem while maintaining the current interface. Basically, I realized that any time we need to check if an attribute is a Mobility attribute in the context of attribute changes, we can do this quickly and easily if we track which attributes have changed by Mobility.

So I add a method mobility_changed_attributes which returns a set, and before calling attribute_will_change!, just add the attribute name to the set. Then we can override _read_attribute as before, except that rather than a tricky regex, we just check if this set contains the attribute, if it does then dispatch to __send__.

This is not beautiful, but it fixes all (currenty) dirty issues in AR 5.2, should have minimal performance impact (also since it uses def instead of define_method, and only includes the ReadAttribute method once), and of course only impacts users who use the dirty plugin.

I tried to think about how we could do things differently without fallthrough accessors, but the issue is that there is a case where the user (say) does not use locale accessors, and does not specify available_locales. In this case, we really don't know what locales we should be matching against. Fallthrough accessors allow this case to be handled seamlessly, although at a performance cost (which I will mention in the readme).

Also, regardless, any solution I can imagine will have to override _read_attribute, given how AR now overrides AM::Dirty methods. The only way around this would be to explicitly add attributes to the @attributes hash, which seems like it would be asking for yet more problems.

So... if specs pass (locally only remaining failures are with the generators), I'd like to merge this and release in 0.3.0 with the changes, so we're (mostly) prepared for the Rails 5.2 launch.

[shioyama]

Generator spec failures are from an issue in the specs, not the generators themselves. So basically this PR should fix all known issues with Rails 5.2.

[pwim]

Cool. Took a look at it, and nothing jumps out at me.