Add Message encoder to just log the log message

This encoder is useful for local consumption of the raw log stream, e.g.
during development where the developer might not care for any additional
fields. With this encoder, the log output can mostly resemble a "classic"
line-based log feed.

lib/rackstash/encoders/message.rb

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+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+module Rackstash
+  module Encoders
+    # The Message encoder only returns the message of log event. All other
+    # fields and tags are ignored.
+    #
+    # This encoder is useful in environments where the added fields are not
+    # required, mostly during development where debug logs are directly consumed
+    # by humans
+    class Message
+      # @param event [Hash] a log event as produced by the {Flow}
+      # @return [String] the `"message"` field of the event. Trailing whitespace
+      #     will be removed.
+      def encode(event)
+        event[FIELD_MESSAGE].rstrip
+      end
+    end
+  end
+end

spec/rackstash/encoders/message_spec.rb

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+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'spec_helper'
+
+require 'rackstash/encoders/message'
+
+describe Rackstash::Encoders::Message do
+  let(:encoder) { Rackstash::Encoders::Message.new }
+
+  describe '#encode' do
+    it 'gets the message from the event hash' do
+      event = { 'hello' => 'world', 'message' => 'hello' }
+      expect(encoder.encode(event)).to eql 'hello'
+    end
+
+    it 'rstrips the message' do
+      event = { 'message' => "\n\t \nline1\nline2\n  \n\t\n" }
+      expect(encoder.encode(event)).to eql "\n\t \nline1\nline2"
+    end
+  end
+end