Freeze Messages directly after initialization

This ensures that they are actually immutable.

lib/rackstash/message.rb

@@ -53,6 +53,8 @@ module Rackstash
       @progname = dup_freeze(progname)
 
       @message = cleanup(message)
+
+      freeze
     end
 
     # @return [String] the guman readable label for the {#severity}.

spec/rackstash/message_spec.rb

@@ -50,6 +50,10 @@ describe Rackstash::Message do
       expect(message.message).not_to equal str
       expect(message.message).to eql str
     end
+
+    it 'freezes the Message' do
+      expect(described_class.new('message')).to be_frozen
+    end
   end
 
   describe '#message' do