NullIO#closed should return false (puma#2883)

lib/puma/null_io.rb

@@ -53,8 +53,9 @@ def flush
       self
     end
 
+    # This is used as singleton class, so can't have state.
     def closed?
-      true
+      false
     end
   end
 end

test/test_null_io.rb

@@ -65,7 +65,7 @@ def test_flush_returns_self
     assert_equal nio, nio.flush
   end
 
-  def test_closed_returns_true
-    assert_equal true, nio.closed?
+  def test_closed_returns_false
+    assert_equal false, nio.closed?
   end
 end