Nested class sequences are not flattened

[zlorf]

Recently we upgraded our dependencies, switching from sablono 0.7.1 to 0.8.6.

And because of 17ec99d, some our code stopped working.

That is because previously (join-classes ["a" ["x" ["y"]]]) returned
"a x y"
and now
"a x [\"y\"]"

flatening happens only at the first level, what is problematic, when you have something like

(html [:div.a {:class [(if foo "x") (get-classes)]}])

where get-classes returns a dynamic list/vector.