Use ActiveModel's normalize to strip records

[liepauls]

Hi and thanks for maintaining this gem!

Here's a potential implementation of normalize to strip records (#67).

There are two caveats:

1. ActiveModel now has to be used instead of ActiveAttr to build test objects. Also plain ActiveAttr models lack .normalize method, thus aren't supported anymore.
2. if and unless options are no longer supported. normalize doesn't have those options and the record isn't available in the normalize context. I guess this kinda makes sense, because without this option, we're normalizing the data consistently across all instances of a specific model. However it sucks that the gem's API is changed

If normalize approach is something we want to continue with, there are few things left for me to do:

1. Add some tests (maybe test that #save(validate: false) works, etc.)
2. Remove if and unless from VALID_OPTIONS, and delete tests for if and unless options
3. Update readme

Feel free to question, criticize and comment 🙏

[liepauls]

@rmm5t have you had the chance to look at the PR?