[w3c/DOM-Parsing] It's possible that 'retrieve a preferred prefix string' returns a wrong prefix for the specified namespace (#45)

https://w3c.github.io/DOM-Parsing/#dfn-retrieving-a-preferred-prefix-string
WPT domparsing/XMLSerializer-serializeToString.html has a test case for this behavior.```"Check if an attribute with namespace and no prefix is serialized with the nearest-declared prefix even if the prefix is assigned to another namespace."```

```
const root = (new DOMParser()).parseFromString('<el1 xmlns:p="u1" xmlns:q="u1"><el2 xmlns:q="u2"/></el1>', 'text/xml').documentElement;
root.firstChild.setAttributeNS('u1', 'name', 'v');
console.log((new XMLSerializer()).serializeToString(root));
```
If we follow the current specification, 'retrieve a preferred prefix string' for ```name``` attribute choses ```q``` prefix because ```q``` is in the list for ```u1``` namespace. However, namespace for ```q``` is ```u2``` on the element.  The serialization result would be ```<el1 ...><el2 xmlns:q="u2" q:name="v"/></el1>```

Before we [add](https://w3c.github.io/DOM-Parsing/#dfn-add) prefix and namespace to a map, we need to remove the prefix associated to another namespace from the map.




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https://github.com/w3c/DOM-Parsing/issues/45