NBSP erroneously treated as generic whitespace

`<span>foo &nbsp;&middot;&nbsp; bar</span>` is being compressed down to `<span>foo &middot; bar</span>`, as if `&nbsp;` were just generic whitespace and not a significant character. This is wrong; `&nbsp;` should be preserved.

I am just using the default config (`new HtmlMin()->minify($html)`).