Fix Style/CaseLikeIf not properly handling overriden equality methods

[Skipants]

If a class overrides one of #==, #eql?, or equal? and the overridden method has no arguments, then CaseLikeIf#find_target_in_equality_node would error as it assumes there is at least one argument.

Without the fix, the new spec returns an error:

➜  rubocop (master) ✔ bundle exec rspec spec/rubocop/cop/style/case_like_if_spec.rb:340
Run options: include {:focus=>true, :locations=>{"./spec/rubocop/cop/style/case_like_if_spec.rb"=>[340]}}

Randomized with seed 42434
F

Failures:

  1) RuboCop::Cop::Style::CaseLikeIf does not register an offense when an object overrides `equal?` with no arity
     Failure/Error: if argument.literal? || const_reference?(argument)

     NoMethodError:
       undefined method `literal?' for nil:NilClass
     # ./lib/rubocop/cop/style/case_like_if.rb:121:in `find_target_in_equality_node'
     # ./lib/rubocop/cop/style/case_like_if.rb:105:in `find_target_in_send_node'
     # ./lib/rubocop/cop/style/case_like_if.rb:95:in `find_target'
     # ./lib/rubocop/cop/style/case_like_if.rb:39:in `on_if'
     # ./lib/rubocop/cop/commissioner.rb:99:in `block (2 levels) in trigger_responding_cops'
     # ./lib/rubocop/cop/commissioner.rb:152:in `with_cop_error_handling'
     # ./lib/rubocop/cop/commissioner.rb:98:in `block in trigger_responding_cops'
     # ./lib/rubocop/cop/commissioner.rb:97:in `each'
     # ./lib/rubocop/cop/commissioner.rb:97:in `trigger_responding_cops'
     # ./lib/rubocop/cop/commissioner.rb:70:in `on_if'
     # ./lib/rubocop/cop/commissioner.rb:85:in `investigate'
     # ./lib/rubocop/cop/team.rb:152:in `investigate_partial'
     # ./lib/rubocop/cop/team.rb:83:in `investigate'
     # ./lib/rubocop/rspec/cop_helper.rb:54:in `_investigate'
     # ./lib/rubocop/rspec/cop_helper.rb:25:in `inspect_source'
     # ./lib/rubocop/rspec/expect_offense.rb:187:in `expect_no_offenses'
     # ./spec/rubocop/cop/style/case_like_if_spec.rb:341:in `block (2 levels) in <top (required)>'

Finished in 0.03323 seconds (files took 1.35 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./spec/rubocop/cop/style/case_like_if_spec.rb:340 # RuboCop::Cop::Style::CaseLikeIf does not register an offense when an object overrides `equal?` with no arity

Randomized with seed 42434

I doubt you'll see much of == being overridden in the wild, but this particular case was found with equal?. An object in my work's app takes in a bunch of data in its constructor and equal? is overridden and used to verify some criterion on it.

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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[koic]

Nice patch! Thank you!