Serialize unrecognized namespaces according to the prefix used

[kmcallister]

We have to query this information from the DOM.

Offshoot of #14.

cc @SimonSapin

[kmcallister]

As a specific example, the element created by

document.createElementNS("http://example.com/ns", "foo:bar")

should serialize as

<foo:bar></foo:bar>

even though its localName is only bar.

[SimonSapin]

Perhaps an optional prefix should be part of string_cache::namespace::QualName

[Ms2ger]

Exactly: servo/string-cache#26