backgroundPositions should concatenate values without a comma

[TomiS]

At the moment

CssJs.backgroundPositions([#right, #top])

produces CSS

background-position: right, top;

I believe it should concatenate the values without a comma like this

background-position: right top;

[giraud]

You can have comma if you position multiple backgrounds (from mdn):

but, maybe it should be a list of list instead.
It would be great if you can add a PR for the fix