Return appropriate tuple for string-set:none

[Tontyna]

This snippet used to crash WeasyPrint with UnboundLocalError: local variable 'computed_value' referenced before assignment:

<style>
div {
  string-set: none;
}
</style>
<div>OMG</div>

Reason being the tuple-constructor in computed_values.string_set() splitting 'none' instead of a tuple consiting of a string-name and its string-value.

BTW: WeasyPrint's unconventional way of handling string-set enables declarations like string-set: none, stringname "value";.

[liZe]

Of course, thank you