Ensure the stored object_type of a ClassRegistry is always frozen

lib/rackstash/class_registry.rb

@@ -16,7 +16,7 @@ module Rackstash
     # @param object_type [#to_s] the human-readable singular name of the
     #   registered objects. It is used to build more useful error messages.
     def initialize(object_type = 'class')
-      @object_type = object_type.to_s
+      @object_type = object_type.to_s.dup.freeze
       @registry = {}
     end
 

lib/rackstash/filter.rb

@@ -52,7 +52,7 @@ module Rackstash
       # @return [ClassRegistry] the {ClassRegistry} object which allows to
       #   register and retrieve available filter classes
       def registry
-        @registry ||= Rackstash::ClassRegistry.new('filter'.freeze)
+        @registry ||= Rackstash::ClassRegistry.new('filter')
       end
 
       # Create a new filter instance from the specified class and the given