[cssom] Don't serialize shorthand if importance differs

[upsuper]

For the following declarations:

div {
  background: green;
  background-image: none !important;
}

the current spec should return something for getPropertyValue('background'), but all browsers (even Servo) agree on that this should return nothing because the importance of its longhands differ.

This PR fixes this issue.

It can alternatively be fixed in "serialize a CSS value" algorithm. Given that the other callsite of this algorithm with a list (which is "serialize a CSS declaration block") already handles important flag itself, I guess it is better being put in getPropertyValue rather than "serialize a CSS value".

[FremyCompany]

That makes sense to me.

[zcorpan]

Thanks, looks good. Do you have a test case you can submit to web-platform-tests or csswg-test also? :-)

[upsuper]

There already exists a web-platform test covers this.