Writes should always clear the query cache

It shouldn't matter whether or not QC is enabled, inserts, updates, etc
(anything that modifies the state of the DB) should invalidate the
cache.

activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb

@@ -18,7 +18,7 @@ def dirties_query_cache(base, *method_names)
           method_names.each do |method_name|
             base.class_eval <<-end_code, __FILE__, __LINE__ + 1
               def #{method_name}(*)
-                ActiveRecord::Base.clear_query_caches_for_current_thread if @query_cache_enabled
+                ActiveRecord::Base.clear_query_caches_for_current_thread
                 super
               end
             end_code

activerecord/test/cases/query_cache_test.rb

@@ -40,6 +40,25 @@ def teardown
     super
   end
 
+  def test_writes_should_always_clear_cache
+    assert_cache :off
+
+    mw = middleware { |env|
+      Post.first
+      query_cache = ActiveRecord::Base.connection.query_cache
+      assert_equal 1, query_cache.length, query_cache.keys
+      Post.connection.uncached do
+        # should clear the cache
+        Post.create!(title: "a new post", body: "and a body")
+      end
+      query_cache = ActiveRecord::Base.connection.query_cache
+      assert_equal 0, query_cache.length, query_cache.keys
+    }
+    mw.call({})
+
+    assert_cache :off
+  end
+
   def test_exceptional_middleware_clears_and_disables_cache_on_error
     assert_cache :off