Fix incorrect usage of named arguments

When some arguments are passed to `#as_html`, `method` will be always nil.

``` irb
>> '<h1>hi</h1>'.as_html(with: :w3m)
ArgumentError: unexpected method 
        from /home/ursm/src/as_foo/lib/as_foo/as_html.rb:10:in `as_html'
```