Always quote attrs ending with a slash.

This fixes a bug I hit:

    <a href="/foo/">Foo</a>

Which got minified to:

    <a href=/foo/>Foo</a>

But got interpreted by Webkit as

    <a href=/foo/></a>Foo

Because it confused the `/>` for a self-closing `a` element.
Add basic test case.
Express the behaviour in a test.
Make sure tags don't get closed accidentally.
Merge pull request #12 from nvie/patch-1

Always quote attrs ending with a slash.