When we encounter an URL with multiple @'s, the first one should be treated

When we encounter an URL with multiple @'s, the first one should be treated
as the username separator and the subsequent ones automatically escaped.

Therefore, `http://a@b@c.cz/' is normalized as `http://a@b%40c.cz/'.
It's probably an invalid URL (have to check all the possible loopholes
in the spec yet :) ), but it occurs in the wild anyway, so let's try
to treat them with grace.