Use a class instance for Message's default formatter

Calling #call in a custom class instead of a lambda performs much
better:

    require 'benchmark/ips'

    class RawFormatter
      def call(_severity, _timestamp, _progname, msg) msg end
    end

    Benchmark.ips do |x|
      message = 'my message'

      proc = ->(_severity, _timestamp, _progname, msg) { msg }
      instance = RawFormatter.new

      x.report('proc') do
        proc.call(nil, nil, nil, message)
      end

      x.report('instance') do
        instance.call(nil, nil, nil, message)
      end

      x.compare!
    end

    # Warming up --------------------------------------
    #                 proc   159.882k i/100ms
    #             instance   182.648k i/100ms
    # Calculating -------------------------------------
    #                 proc      4.612M (± 5.2%) i/s -     23.023M in   5.005950s
    #             instance      7.716M (± 6.3%) i/s -     38.539M in   5.015306s
    #
    # Comparison:
    #             instance:  7716097.7 i/s
    #                 proc:  4611920.5 i/s - 1.67x  slower

lib/rackstash/formatter.rb

@@ -25,4 +25,10 @@ module Rackstash
       "#{msg2str(msg)}\n"
     end
   end
+
+  class RawFormatter
+    def call(_severity, _timestamp, _progname, msg)
+      msg
+    end
+  end
 end

lib/rackstash/message.rb

@@ -6,7 +6,7 @@
 module Rackstash
   # This class and all its data are immutable after initialization
   class Message
-    RAW_FORMATTER = ->(_severity, _timestamp, _progname, msg) { msg }
+    RAW_FORMATTER = RawFormatter.new
 
     SEVERITY_LABEL = [
       'DEBUG'.freeze,

spec/rackstash/formatter_spec.rb

@@ -46,3 +46,12 @@ describe Rackstash::Formatter do
     expect(formatter.call('ERROR', Time.now, 'progname', object)).to eql "#{inspected}\n"
   end
 end
+
+describe Rackstash::RawFormatter do
+  let(:formatter) { Rackstash::RawFormatter.new }
+
+  it 'returns the message' do
+    msg = 'my message'
+    expect(formatter.call('ERROR', Time.now, 'progname', msg)).to equal msg
+  end
+end