Revert "Properly cache value when it is "false""

This reverts commit 551f313.

Reason: see
rails#2010 (comment)

activesupport/lib/active_support/cache.rb

@@ -555,7 +555,7 @@ def initialize(value, options = {})
         @expires_in = options[:expires_in]
         @expires_in = @expires_in.to_f if @expires_in
         @created_at = Time.now.to_f
-        if defined?(value)
+        if value
           if should_compress?(value, options)
             @value = Zlib::Deflate.deflate(Marshal.dump(value))
             @compressed = true
@@ -574,7 +574,7 @@ def raw_value
 
       # Get the value stored in the cache.
       def value
-        if defined?(@value)
+        if @value
           val = compressed? ? Marshal.load(Zlib::Inflate.inflate(@value)) : @value
           unless val.frozen?
             val.freeze rescue nil

activesupport/test/caching_test.rb

@@ -188,11 +188,6 @@ def test_should_read_and_write_nil
     assert_equal nil, @cache.read('foo')
   end
 
-  def test_should_read_and_write_false
-    assert_equal true, @cache.write('foo', false)
-    assert_equal false, @cache.read('foo')
-  end
-
   def test_read_multi
     @cache.write('foo', 'bar')
     @cache.write('fu', 'baz')