Restore ability to deserialize attributes that represents XML namespace mappings (xmlns:xxx)

[Mingun]

Fixes #537.

Change in behavior was made in 8b50c64 of #490

quick-xml/src/de/key.rs

Lines 73 to 80 in 4a09041

	/// Creates deserializer from name of an attribute
	pub fn from_attr(name: QName<'d>, decoder: Decoder) -> Result<Self, DeError> {
	let local = decode_name(name, decoder)?;
	Ok(Self {
	name: Cow::Owned(format!("@{local}")),
	})
	}

Here only local part of an attribute name is mapped to the field name. Because xmlns:... attributes is a special names, I make a special handling for that names -- use the full name of an attribute.

Note, that usually xmlns should not be considered as a data that could be accessed via serde interface. This is metadata of an XML serialization format. When (if) #218 would be implemented, that attributes most likely wouldn't be accessible at all.