Avoid repeated dup of as_json options

Previously for every element in an Array and every value in a Hash we
would duplicate the options hash passed in.

I did this very conservatively, duping and freezing the Hash when it is
not already frozen so that any cases mutating the Hash should be caught.
I don't feel very strongly about this and we could remove this in the
future.

Author: jhawthorn

activemodel/lib/active_model/dirty.rb

@@ -256,7 +256,8 @@ def initialize_dup(other) # :nodoc:
     end
 
     def as_json(options = {}) # :nodoc:
-      options[:except] = [*options[:except], "mutations_from_database", "mutations_before_last_save"]
+      except = [*options[:except], "mutations_from_database", "mutations_before_last_save"]
+      options = options.merge except: except
       super(options)
     end
 

activesupport/lib/active_support/core_ext/object/json.rb

@@ -165,7 +165,8 @@ def as_json(options = nil) # :nodoc:
 class Array
   def as_json(options = nil) # :nodoc:
     if options
-      map { |v| v.as_json(options.dup) }
+      options = options.dup.freeze unless options.frozen?
+      map { |v| v.as_json(options) }
     else
       map { |v| v.as_json }
     end
@@ -189,7 +190,8 @@ def as_json(options = nil) # :nodoc:
 
     result = {}
     if options
-      subset.each { |k, v| result[k.to_s] = v.as_json(options.dup) }
+      options = options.dup.freeze unless options.frozen?
+      subset.each { |k, v| result[k.to_s] = v.as_json(options) }
     else
       subset.each { |k, v| result[k.to_s] = v.as_json }
     end

activesupport/lib/active_support/json/encoding.rb

@@ -36,7 +36,7 @@ def initialize(options = nil)
         # Encode the given object into a JSON string
         def encode(value)
           unless options.empty?
-            value = value.as_json(options.dup)
+            value = value.as_json(options.dup.freeze)
           end
           json = stringify(jsonify(value))