Enable immutable string literals in Ruby

ruby/lib/plus_codes.rb

@@ -1,15 +1,17 @@
+# frozen_string_literal: true
+
 # Plus+Codes is a Ruby implementation of Google Open Location Code (Plus Codes).
 #
 # @author We-Ming Wu
 module PlusCodes
   # The character set used to encode coordinates.
-  CODE_ALPHABET = '23456789CFGHJMPQRVWX'.freeze
+  CODE_ALPHABET = '23456789CFGHJMPQRVWX'
 
   # The character used to pad a code
-  PADDING = '0'.freeze
+  PADDING = '0'
 
   # A separator used to separate the code into two parts.
-  SEPARATOR = '+'.freeze
+  SEPARATOR = '+'
 
   # The max number of characters can be placed before the separator.
   SEPARATOR_POSITION = 8

ruby/lib/plus_codes/code_area.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module PlusCodes
   # [CodeArea] contains coordinates of a decoded Open Location Code(Plus+Codes).
   # The coordinates include the latitude and longitude of the lower left and

ruby/lib/plus_codes/open_location_code.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_relative '../plus_codes'
 require_relative '../plus_codes/code_area'
 

ruby/open-location-code.gemspec

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 lib = File.expand_path('lib', __dir__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'date'

ruby/test/plus_codes_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test/unit'
 require_relative '../lib/plus_codes/open_location_code'