Add test case

Author: Nicholas Stuart

activesupport/test/cache/behaviors/failure_safety_behavior.rb

@@ -9,17 +9,19 @@ def test_fetch_read_failure_returns_nil
     end
   end
 
-  ##
-  # Though the `write` part of fetch fails for the same reason
-  # `read` will, the block result is still executed and returned.
-  def test_fetch_read_failure_returns_block_result
+  def test_fetch_read_failure_does_not_attempt_to_write
     @cache.write("foo", "bar")
 
     emulating_unavailability do |cache|
-      val = cache.fetch("foo") { '1' }
+      val = cache.fetch("foo") { "1" }
 
-      assert_equal '1', val
+      ##
+      # Though the `write` part of fetch fails for the same reason
+      # `read` will, the block result is still executed and returned.
+      assert_equal "1", val
     end
+
+    assert_equal "bar", @cache.read("foo")
   end
 
   def test_read_failure_returns_nil