[ruby/singleton] Simplify implementation of `Singleton#instance`.

(https://github.com/ruby/singleton/pull/9)

- Add more tests to cover rails' usage.

lib/singleton.rb

@@ -121,12 +121,7 @@ module Singleton
     end
 
     def instance # :nodoc:
-      return @singleton__instance__ if @singleton__instance__
+      @singleton__instance__ || @singleton__mutex__.synchronize { @singleton__instance__ ||= new }
-      @singleton__mutex__.synchronize {
-        return @singleton__instance__ if @singleton__instance__
-        @singleton__instance__ = new()
-      }
-      @singleton__instance__
     end
 
     private

test/test_singleton.rb

@@ -94,6 +94,23 @@ class TestSingleton < Test::Unit::TestCase
     assert_same a, b
   end
 
+  def test_inheritance_creates_separate_singleton
+    a = SingletonTest.instance
+    b = Class.new(SingletonTest).instance
+
+    assert_not_same a, b
+  end
+
+  def test_inheritance_instantiation
+    klass = Class.new do
+      include Singleton
+
+      public_class_method :new
+    end
+
+    assert Class.new(klass).new
+  end
+
   def test_class_level_cloning_preserves_singleton_behavior
     klass = SingletonTest.clone
 
@@ -101,4 +118,8 @@ class TestSingleton < Test::Unit::TestCase
     b = klass.instance
     assert_same a, b
   end
+
+  def test_class_level_cloning_creates_separate_singleton
+    assert_not_same SingletonTest.instance, SingletonTest.clone.instance
+  end
 end