Ensure that the Formatter freezes the strings in all cases

Previously, the returned String would been frozen only in Rubies
understanding the frozen_literals magic comment.

lib/rackstash/formatter.rb

@@ -23,7 +23,7 @@ module Rackstash
     # @param msg [String, Exception, #inspect] the log message
     # @return [String] the formatted message with a final newline character
     def call(_severity, _time, _progname, msg)
-      "#{msg2str(msg)}\n"
+      "#{msg2str(msg)}\n".freeze
     end
   end
 

spec/rackstash/formatter_spec.rb

@@ -13,15 +13,22 @@ describe Rackstash::Formatter do
   let(:formatter) { described_class.new }
 
   it 'formats plain strings' do
-    expect(formatter.call('ERROR', Time.now, 'progname', 'Hello')).to eql "Hello\n"
+    expect(formatter.call('ERROR', Time.now, 'progname', 'Hello'))
+      .to eql("Hello\n")
+      .and be_frozen
   end
 
   it 'formats stringifiable objects' do
-    expect(formatter.call('ERROR', Time.now, 'progname', 123)).to eql "123\n"
+    expect(formatter.call('ERROR', Time.now, 'progname', 123))
+      .to eql("123\n")
+      .and be_frozen
   end
 
   it 'formats Arrays' do
-    expect(formatter.call('ERROR', Time.now, 'progname', [1, 'y'])).to eql "[1, \"y\"]\n"
+    expect(formatter.call('ERROR', Time.now, 'progname', [1, 'y']))
+      .to eql("[1, \"y\"]\n")
+      .and be_frozen
+
   end
 
   it 'formats exceptions' do
@@ -36,15 +43,19 @@ describe Rackstash::Formatter do
       \\AAn Error \\(StandardError\\)
       #{Regexp.escape __FILE__}:#{__LINE__ - 7}:in `block .*`
     EOF
-    expect(formatter.call('ERROR', Time.now, 'progname', exception)).to match checker
-  end
+    expect(formatter.call('ERROR', Time.now, 'progname', exception))
+      .to match(checker)
+      .and be_frozen
+    end
 
   it 'inspects unknown objects' do
     object = Object.new
     inspected = object.inspect
 
     expect(object).to receive(:inspect).once.and_call_original
-    expect(formatter.call('ERROR', Time.now, 'progname', object)).to eql "#{inspected}\n"
+    expect(formatter.call('ERROR', Time.now, 'progname', object))
+      .to eql("#{inspected}\n")
+      .and be_frozen
   end
 end