add unless_exist option to memory store

activesupport/lib/active_support/cache/memory_store.rb

@@ -137,6 +137,7 @@ def read_entry(key, options) # :nodoc:
         def write_entry(key, entry, options) # :nodoc:
           synchronize do
             old_entry = @data[key]
+            return false if @data.key?(key) && options[:unless_exist]
             @cache_size -= old_entry.size if old_entry
             @cache_size += entry.size
             @key_access[key] = Time.now.to_f

activesupport/test/caching_test.rb

@@ -684,6 +684,13 @@ def @cache.delete_entry (*args)
     assert @cache.exist?(2)
     assert !@cache.exist?(1)
   end
+
+  def test_write_with_unless_exist
+    assert_equal true, @cache.write(1, "aaaaaaaaaa")
+    assert_equal false, @cache.write(1, "aaaaaaaaaa", :unless_exist => true)
+    @cache.write(1, nil)
+    assert_equal false, @cache.write(1, "aaaaaaaaaa", :unless_exist => true)
+  end
 end
 
 uses_memcached 'memcached backed store' do