Fix an incorrect auto-correct for Performance/RedundantSplitRegexpArgument

[koic]

Follow #190.

This PR fixes the following incorrect auto-correct for Performance/RedundantSplitRegexpArgument when using consecutive special string chars.

% cat example.rb
"foo\nbar\nbaz\n".split(/\n\n/)

% bundle exec rubocop example.rb --only Performance/RedundantSplitRegexpArgument -a
Inspecting 1 file
C

Offenses:

example.rb:1:25: C: [Corrected]
Performance/RedundantSplitRegexpArgument: Use string as argument instead of regexp.
"foo\nbar\nbaz\n".split(/\n\n/)
                        ^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

## Before

```console
% cat example.rb
"foo\nbar\nbaz\n".split("nn")

After

% cat example.rb
"foo\nbar\nbaz\n".split("\n\n")

---

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• Added tests.
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