[bug] explore removing JRuby implementation's namespace cache

[flavorjones]

Please describe the bug

It's not clear to me why we're using a namespace cache in the JRuby implementation. It is causing problems in edge cases like #1247 that I think would be better solved by looking up the node's parent chain for an identical namespace definition a) when it is added, and b) when the node is reparented.

This is what we do in the CRuby implementation. It's more complicated (and likely still has some bugs) but it is obviously more correct in these edge cases.

An example taken from #1247 is:

  it "handles multiple instances of a namespace definition" do
    # this describes behavior that is broken in JRuby related to the namespace cache and should be fixed
    doc = Nokogiri::XML::Document.parse("<root>")
    child1 = doc.create_element("a", "xmlns:foo" => "http://nokogiri.org/ns/foo")
    assert_equal(1, child1.namespace_definitions.length)
    child1.namespace_definitions.first.tap do |ns|
      assert_equal("foo", ns.prefix)
      assert_equal("http://nokogiri.org/ns/foo", ns.href)
    end

    child2 = doc.create_element("b", "xmlns:foo" => "http://nokogiri.org/ns/foo")
    pending_if("broken in JRuby related to the namespace cache and should be fixed", Nokogiri.jruby?) do
      assert_equal(1, child2.namespace_definitions.length)
      child2.namespace_definitions.first.tap do |ns|
        assert_equal("foo", ns.prefix)
        assert_equal("http://nokogiri.org/ns/foo", ns.href)
      end
    end
  end

In summary, the second node does not have a namespace definition in the JRuby implementation. This is clearly wrong.

A partial fix would be to reimplement Node#namespace_definitions to look at the attributes for a namespace definition:

  @JRubyMethod
  public RubyArray<?>
  namespace_definitions(ThreadContext context)
  {
    // TODO: I think this implementation would be better but there are edge cases
    RubyArray<?> nsdefs = RubyArray.newArray(context.getRuntime());
    NamedNodeMap attrs = node.getAttributes();
    for (int j = 0 ; j < attrs.getLength() ; j++) {
      Attr attr = (Attr)attrs.item(j);
      if ("http://www.w3.org/2000/xmlns/" == attr.getNamespaceURI()) {
        nsdefs.append(XmlNamespace.createFromAttr(context.getRuntime(), attr));
      }
    }
    return nsdefs;
  }

but we also need to then implement something like xml_node.c:relink_namespace in order to make this match the CRuby behavior.