Fix declaring node also allows to use prefix for namespace

[Fruchtzwerg94]

Issue / problem

Writing a SVG file where the namespace of a node is declared as an attribute in this node fails to resolve the namespace prefix.

As a minimal example, assume we want to add some additional meta data to a SVG file:

SvgDocument svg = SvgDocument.Open("MyImageWithMetaData.svg");

const string NAMESPACE_RDF = "https://www.w3.org/1999/02/22-rdf-syntax-ns#";
const string NAMESPACE_DC = "https://purl.org/dc/elements/1.1/";

SvgUnknownElement metadata = new SvgUnknownElement("metadata");
NonSvgElement rdfMetadata = new NonSvgElement("RDF", NAMESPACE_RDF);
rdfMetadata.Namespaces["rdf"] = NAMESPACE_RDF;

NonSvgElement rdfMetadataDescription = new NonSvgElement("Description", NAMESPACE_RDF);
rdfMetadataDescription.Children.Add(new NonSvgElement("creator", NAMESPACE_DC) { Content = "Hello World" });
rdfMetadata.Children.Add(rdfMetadataDescription);
metadata.Children.Add(rdfMetadata);
svg.Children.Add(metadata);

svg.Write("MyImageWithMetaData.svg");

The additional meta data will look like this:

<RDF xmlns:rdf="https://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="https://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <rdf:Description />
</RDF>

Obviously the namespace here is mentioned twice which is not necessary since (see e.g. Oracle or w3resource)

The scope of a declared namespace begins at the element where it is declared and applies to the entire content of that element

which means namespace prefixes can be already used in the node they are declared.

<rdf:RDF xmlns:rdf="https://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <rdf:Description />
</rdf:RDF>

This PR also takes care of this and looks up the namespaces declared in the nodes themselfes.

[mrbean-bremen]

Thanks, looks good! Please also add an entry in the release notes, and if possible, a test.

[Fruchtzwerg94]

Thanks, looks good! Please also add an entry in the release notes, and if possible, a test.

Test and changelog added @mrbean-bremen . Please keep in mind that the version and date in the changelog is just a placeholder XX yet and needs to be set accordingly at release.

[mrbean-bremen]

Thanks!