Quote terminal attribute value for unary slash elements

Fixes #233
If it's `<img src="foo" title="foo"/>` it will remove quotes from "src", right (since it's not the last attribute)? It could make sense to add test for that particular case.

Also, as far as attribute goes — we're keeping quotes around all of them, not just "src" (based on what I see in your changes), right? Do you think we should perhaps check attribute name or is that a path to more issues in the future?