[performance] set default KeyFormatter to nil

[quake]

benchmark code

class TestKey
  def initialize(key_formatter)
    @key_formatter = key_formatter
  end

  def key1(key)
    @key_formatter ? @key_formatter.format(key) : key.to_s
  end

  def key2(key)
    @key_formatter.format(key)
  end
end


a = TestKey.new(nil)
b = TestKey.new(Jbuilder::KeyFormatter.new)

called = 10000000
Benchmark.benchmark do |x|
  keys = (1..100).map{|i| "key#{i}"}
  x.report("without formatter:")   { called.times{ a.key1(keys.sample) } }
  x.report("with formatter:")      { called.times{ b.key2(keys.sample) } }
end

result

without formatter:  1.910000   0.000000   1.910000 (  1.918297)
with formatter:  3.030000   0.010000   3.040000 (  3.047730)