Fix deprecation warning about passing strings to middleware
Fix deprecation warning about passing strings to middleware

Passing middleware as strings or symbols is now deprecated.

```
DEPRECATION WARNING: Passing strings or symbols to the middleware
builder is deprecated, please change
them to actual class references.  For example:

  "ActionDispatch::XmlParamsParser" => ActionDispatch::XmlParamsParser
```

rails/actionpack-xml_parser,eileencodes/actionpack-xml_parser