[cssom] serialising declarations with multiple components doesn't work

[gsnedders]

See https://drafts.csswg.org/cssom/#serializing-css-values.

Given a CSS declaration of quotes: 'a' 'b:

Step 1 does nothing. Step 2 creates a list. Step 3 calls serialize a CSS component value with 'a' 'b'. This is nonsense because a b is not a component.

[fantasai]

@gsnedders Fixed in 599e204 !