Add autocorrect support for RSpec/SubjectDeclaration

[eugeneius]

This cop's suggestions can be safely autocorrected.

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[bquorning]

IIRC the resulting code would cause the NamedSubject cop to give offense.

[pirj]

@bquorning let(:subject) { … } -> subject { … } is fine, we only care if several cops would produce opposite or circular corrections.