Add Message#copy_with to create an adjusted copy of an exising message

You can overwrite any (or none) of the existing fields to create a new
frozen copy of a message object.

lib/rackstash/message.rb

@@ -58,6 +58,20 @@ module Rackstash
       freeze
     end
 
+    # Create a new Message object based on the values in `self`, optionally
+    # overwriting any of the them.
+    #
+    # @param (see #initialize)
+    # @return [Message] a new Message
+    def copy_with(message = nil, severity: nil, time: nil, progname: nil)
+      self.class.new(
+        message.nil? ? self.message : message,
+        severity: severity.nil? ? self.severity : severity,
+        time: time.nil? ? self.time : time,
+        progname: progname.nil? ? self.progname : progname
+      )
+    end
+
     # @return [String] the human readable label for the {#severity}.
     # @see Rackstash.severity_label
     def severity_label

spec/rackstash/message_spec.rb

@@ -12,6 +12,9 @@ require 'json'
 require 'rackstash/message'
 
 describe Rackstash::Message do
+  let(:message_args) { {} }
+  let(:message) { described_class.new 'message', **message_args }
+
   describe '#initialize' do
     it 'encodes the message as UTF-8' do
       utf8_str = 'Dönerstraße'
@@ -56,6 +59,35 @@ describe Rackstash::Message do
     end
   end
 
+  describe '#copy_with' do
+    it 'creates a new message instance' do
+      expect(message.copy_with).to be_instance_of described_class
+      expect(message.copy_with.message).to equal message.message
+      expect(message.copy_with.severity).to equal message.severity
+      expect(message.copy_with.progname).to equal message.progname
+      expect(message.copy_with.time).to equal message.time
+
+      expect(message.copy_with).not_to equal message
+    end
+
+    it 'can overwrite the message' do
+      expect(message.copy_with('new stuff').message).to eql 'new stuff'
+    end
+
+    it 'can overwrite the severity' do
+      expect(message.copy_with(severity: 3).severity).to eql 3
+    end
+
+    it 'can overwrite the progname' do
+      expect(message.copy_with(progname: 'blar').progname).to eql 'blar'
+    end
+
+    it 'can overwrite the progname' do
+      time = Time.now.freeze
+      expect(message.copy_with(time: time).time).to equal time
+    end
+  end
+
   describe '#message' do
     it 'is aliased to to_str' do
       message = described_class.new('hello world')