Plain css node attr_* properties should be strings

[bartveneman]

a[href="test"] {}

StyleSheet

    type: 1
    children:
        Rule
            type: 2
            children:
                SelectorList
                    type: 20
                    children:
                        Selector
                            type: 5
                            name: "a[href=\"test\"]"
                            children:
                                TypeSelector
                                    type: 21
                                    name: "a"
                                AttributeSelector
                                    type: 24
                                    name: "href"
                                    value: "\"test\""
                                    attr_operator: 1
                                    attr_flags: 0
                Block
                    type: 7