Fix a false positive for Style/RedundantCondition

[koic]

This PR fixes the following incorrect autocorrect for Style/RedundantCondition when using assignment by hash key access.

Before

The result of auto-correction is syntax error.

% cat example.rb
if @columns_cache[table_name]
  @columns_cache[table_name]
else
  @columns_cache[table_name] = super(table_name)
end

% bundle exec rubocop -a --only Style/RedundantCondition
(snip)

Inspecting 1 file
C

Offenses:

example.rb:1:1: C: [Corrected] Style/RedundantCondition: Use double
pipes || instead.
if @columns_cache[table_name] ...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

% cat example.rb
@columns_cache[table_name] || []=(table_name, super(table_name))

% ruby -c example.rb
example.rb:1: syntax error, unexpected '=', expecting end-of-input
...olumns_cache[table_name] || []=(table_name, super(table_name...

After

The cop accepts the example case.

---

Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[dvandersluis]

Not sure if your example makes perfect sense here, because it should be @columns_cache[table_name] ||= super(table_name) since it was originally a setter, right? I'm not sure if setters are handled by this cop (doesn't appear so from the tests), another one, or not at all.

[koic]

My example was incorrect and your feedback is perfectly right. In any case, there was an incompatible auto-correction, and first of all, I made it possible to accept the example case where hash assignment setter is used. I updated this PR. Thank you for your review!