Proposal: Make it easier to define Top-Level Namespaces with prefix

[thoro]

First of, works amazingly, and also generates exactly what the encoding/xml should have been doing from the beginning.

Currently to include top-level namespaces with a prefix you'd have to do the following:

type DidlLite struct {
	XMLName xml.Name `xml:"urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/ DIDL-Lite"`
	UPNP string `xml:"http://www.w3.org/2000/xmlns/ xmlns:upnp,attr"`
	DC string `xml:"http://www.w3.org/2000/xmlns/ xmlns:dc,attr"`
	Items []Item `xml:"urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/ item"`
}

DidlLite{
  UPNP: "urn:schemas-upnp-org:metadata-1-0/upnp/",
  DC: "http://purl.org/dc/elements/1.1/",
}

Which means that the struct is not useable without an initializer (NewDidlLite) function.

A small change could make defining top-level namespaces more user friendly, but would entail a bit of magic (i.e. implicit knowledge)

type DidlLite struct {
	XMLName xml.Name `xml:"urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/ DIDL-Lite"`
	UPNP string `xml:"urn:schemas-upnp-org:metadata-1-0/upnp/ xmlns:upnp,attr"`
	DC string `xml:"http://purl.org/dc/elements/1.1/ xmlns:dc,attr"`
	Items []Item `xml:"urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/ item"`
}

This would pre-encode the namespace for the prefix inside the xml tags.

The change in marshal.go, L845 could look like that:

	if prefix == xmlnsPrefix && attr.Name.Space != xmlnsURL {
		if prefix, prefixCreated := p.createPrefix(attr.Name.Space, local); prefixCreated {
			p.writePrefixAttr(prefix, attr.Name.Space)
		}
		continue
	}

So that it will only write out the prefix and not any additional attribute.

I understand that this functionality might be undesirable, but I landed on it intuitivly before figuring out how else it could be done.