(bug 40834) mw.loader: Fix comma-joined 404 error.

(bug 40834) mw.loader: Fix comma-joined 404 error.

It passed an array instead of a url to addLink so internally
Array.prototype.toString happened which joins by comma.

In debug mode modules with more than 1 stylesheet were 404 error
as it was trying to load 'http://.../foo.css,http://.../bar.css'.

Unit tests didn't catch it because they all only tested an array
with 1 url. Updated the test to assert this from now on.

Follows-up I3e8227ddb87fd9441071ca935439fc6467751dab.

Change-Id: I32a79af637acb782e07bbae413b0f007350096c3