Fixes for errors in Rails/DeprecatedActiveModelErrorsMethods

[BrianHawley]

• Fixed a NoMethodError on nil for errors.keys in a model.
• Fixed a bad autocorrection of errors.details[:name] << value.
  There isn't really a replacement for this one.
• The values, to_h, and to_xml methods are deprecated too.
• Made the range calculation more exact so the replacement is easier.
• Did some refactors prompted by rubocop complaints.
• Fixed a misspelling of autocorrectable.
• Added missing correction assertions to test cases.
• Added missing test cases.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[BrianHawley]

I added 3 changelog entries.

Basically, I went through the ActiveModel::Errors class, comparing Rails versions, and got a list of all the deprecated methods which weren't covered already, and added test cases for them. I also added correction assertions to all the test cases that didn't have them, and checked every correction to make sure that it generated code that would work. Then I fixed the cop class until the tests passed. I got the list of corrections needed from the tests that failed.

There's no check for errors.each do |key, value| because I don't know how to specify that. There's also a test case for which a better correction would exist, but I didn't want to add that in a bug fix PR.

[BrianHawley]

You can do errors.add(:name, value) as a replacement for errors.messages[:name] << value but there's no equivalent action for errors.details[:name] << value without including the message, so it seemed safer to not correct it.

Maybe we should add an example of a correction using errors.add to replace both changing the messages and details?

[BrianHawley]

@koic you did previous work on this class. Do you think you could review?

[koic]

Can you separate the PR for each changelog entry? I think it can be separated into three PRs.

[BrianHawley]

@koic I created #740, #741, and #742. After each is merged, I'll need to rebase the others, for the shared test code.