runtime should be properly initialized.

activerecord/lib/active_record/log_subscriber.rb

@@ -5,7 +5,7 @@ def self.runtime=(value)
     end
 
     def self.runtime
-      Thread.current["active_record_sql_runtime"]
+      Thread.current["active_record_sql_runtime"] ||= 0
     end
 
     def self.reset_runtime
@@ -45,5 +45,4 @@ def logger
   end
 end
 
-ActiveRecord::LogSubscriber.runtime = 0
 ActiveRecord::LogSubscriber.attach_to :active_record
\ No newline at end of file

activerecord/test/cases/log_subscriber_test.rb

@@ -57,4 +57,8 @@ def test_cached_queries_doesnt_log_when_level_is_not_debug
     wait
     assert_equal 0, @logger.logged(:debug).size
   end
+
+  def test_initializes_runtime
+    Thread.new { assert_equal 0, ActiveRecord::LogSubscriber.runtime }.join
+  end
 end