Ensure old buffers get properly cleared to avoid leaking memory

Signed-off-by: NJoshua Peek <josh@joshpeek.com>

activesupport/lib/active_support/buffered_logger.rb

@@ -116,7 +116,7 @@ def buffer
       end
 
       def clear_buffer
-        @buffer[Thread.current] = []
+        @buffer.delete(Thread.current)
       end
   end
 end