Merge pull request #542 from nainar/issue_519

CSSURLImageValue should serialize url with quotes

Author: nainar

css-typed-om/Overview.bs

@@ -2890,7 +2890,7 @@ of serializations of the contained {{CSSTransformComponent}} objects.
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 {{CSSURLImageValue}} objects are serialized to the string given by
-"url(" + {{CSSURLImageValue/url}} + ")".
+'url("' + {{CSSURLImageValue/url}} + '")'.
 
 {{CSSFontFaceValue}} Serialization {#fontfacevalue-serialization}
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