ActiveSupport::Delegation allow to specify the signature

When delegating known APIs, rather that to let Delegator try to
inspect the signature, or to fallback to `...`, we can directly
specify it.

This is both faster and make for nicer delegators that have
the right signature.

Author: byroot

activesupport/lib/active_support/current_attributes.rb

@@ -132,7 +132,8 @@ def attribute(*names, default: nil)
           end
         end
 
-        Delegation.generate(singleton_class, names.flat_map { |name| [name, "#{name}="] }, to: :instance, as: self, nilable: false)
+        Delegation.generate(singleton_class, names, to: :instance, nilable: false, signature: "")
+        Delegation.generate(singleton_class, names.map { |n| "#{n}=" }, to: :instance, nilable: false, signature: "value")
 
         self.defaults = defaults.merge(names.index_with { default })
       end

activesupport/lib/active_support/delegation.rb

@@ -20,7 +20,7 @@ module Delegation # :nodoc:
     RESERVED_METHOD_NAMES = (RUBY_RESERVED_KEYWORDS + %w(_ arg args block)).to_set.freeze
 
     class << self
-      def generate(owner, methods, location: nil, to: nil, prefix: nil, allow_nil: nil, nilable: true, private: nil, as: nil)
+      def generate(owner, methods, location: nil, to: nil, prefix: nil, allow_nil: nil, nilable: true, private: nil, as: nil, signature: nil)
         unless to
           raise ArgumentError, "Delegation needs a target. Supply a keyword argument 'to' (e.g. delegate :hello, to: :greeter)."
         end
@@ -65,7 +65,9 @@ def generate(owner, methods, location: nil, to: nil, prefix: nil, allow_nil: nil
           # Attribute writer methods only accept one argument. Makes sure []=
           # methods still accept two arguments.
           definition = \
-            if /[^\]]=\z/.match?(method)
+            if signature
+              signature
+            elsif /[^\]]=\z/.match?(method)
               "arg"
             else
               method_object = if receiver_class