html.parser.analyzer: when matching on classname, check that the tag has the given class

The methods in html.parser.analyzer that searches for tags of a given class matches using equality on the whole class attribute string. But it mostly doesn't work as one would expect for tags that have multiple classes. E.g

```factor
"<div class=\"date time version\">...</div>" parse-html "version" find-by-class
```
With this patch, the expression returns the given div node.