Disable Performance/Casecmp cop

[onk]

Comparison with downcase or upcase is an idiom.
Idioms have higher priority than some performance improvement.

1.2x faster is not critical.

require "benchmark/ips"
Benchmark.ips do |x|
  a = "foo"
  b = "Foo"
  x.report("downcase") { a.downcase == b }
  x.report("casecmp")  { a.casecmp(b) }
  x.compare!
end

Warming up --------------------------------------
            downcase   179.127k i/100ms
             casecmp   203.340k i/100ms
Calculating -------------------------------------
            downcase      5.126M (±12.2%) i/s -     25.257M in   5.013800s
             casecmp      6.188M (±17.0%) i/s -     29.891M in   5.010809s

Comparison:
             casecmp:  6187759.2 i/s
            downcase:  5125593.1 i/s - same-ish: difference falls within error