Add ActiveModel::Errors#merge!

[jahfer]

Summary

ActiveModel::Errors#merge! allows instances of ActiveModel::Errors to append errors from a separate instance of ActiveModel::Errors onto its own set of errors.

The implementation in this PR does a "deep" merge of the contents within each key so that any keys already containing errors will be merged with the new set, rather than overridden.

This contribution was suggested to me by @rafaelfranca.

Example

person = Person.new
person.errors.add(:name, :blank)

errors = ActiveModel::Errors.new(Person.new)
errors.add(:name, :invalid)

person.errors.merge!(errors)
puts person.errors.messages # => { name: ["can't be blank", "is invalid"] }

[rails-bot]

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[rafaelfranca]

You can remove the `# :nodoc: comment here since we want it to be public API.

[jahfer]

✅

[rafaelfranca]

Are the variable inside the block hashes or arrays? I think they are arrays, so maybe we should rename them?

[jahfer]

Oh they are definitely arrays. Fixed now.