quote StringValue with single quotes, a la QuotedStringValue.__str__

- CSS such as this

    div {
      label: 'a b';
    }

  was being compiled to this

    div{label:a b}

  when this

    div{label:'a b'}

  is what was intended.

scss/__init__.py

@@ -4500,7 +4500,7 @@ def merge(self, obj):
 
 class StringValue(QuotedStringValue):
     def __str__(self):
-        return self.value
+        return "'%s'" % escape(self.value)
 
     def __add__(self, other):
         if isinstance(other, ListValue):