Fix Complex and Rational are duplicable?

See [this test](https://gist.github.com/utilum/78918f1b64f8b61ee732cb266db7c43a).

activesupport/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Change return value of `Rational#duplicable?`, `ComplexClass#duplicable?`
+    to false.
+
+    *utilum*
+
 *   Change return value of `NilClass#duplicable?`, `FalseClass#duplicable?`,
     `TrueClass#duplicable?`, `Symbol#duplicable?` and `Numeric#duplicable?`
     to true with Ruby 2.4+. These classes can dup with Ruby 2.4+.

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

@@ -121,3 +121,23 @@ class Method
     false
   end
 end
+
+class Complex
+  # Complexes are not duplicable:
+  #
+  # Complex(1).duplicable? # => false
+  # Complex(1).dup         # => TypeError: can't copy Complex
+  def duplicable?
+    false
+  end
+end
+
+class Rational
+  # Rationals are not duplicable:
+  #
+  # Rational(1).duplicable? # => false
+  # Rational(1).dup         # => TypeError: can't copy Rational
+  def duplicable?
+    false
+  end
+end

activesupport/test/core_ext/object/duplicable_test.rb

@@ -5,10 +5,10 @@ require "active_support/core_ext/numeric/time"
 
 class DuplicableTest < ActiveSupport::TestCase
   if RUBY_VERSION >= "2.4.0"
-    RAISE_DUP = [method(:puts)]
+    RAISE_DUP = [method(:puts), Complex(1), Rational(1)]
     ALLOW_DUP = ["1", Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal.new("4.56"), nil, false, true, :symbol, 1, 2.3]
   else
-    RAISE_DUP = [nil, false, true, :symbol, 1, 2.3, method(:puts)]
+    RAISE_DUP = [nil, false, true, :symbol, 1, 2.3, method(:puts), Complex(1), Rational(1)]
     ALLOW_DUP = ["1", Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal.new("4.56")]
   end