fix(cruby): reconciliation of ns on unparented nodes

Closes #3459

Author: flavorjones

ext/nokogiri/xml_node.c

@@ -1334,6 +1334,10 @@ set_namespace(VALUE self, VALUE namespace)
 
   xmlSetNs(node, ns);
 
+  if (ns && ns->prefix)  {
+    xmlReconciliateNs(node->doc, node);
+  }
+
   return self;
 }
 

test/xml/test_node.rb

@@ -1336,6 +1336,19 @@ def test_namespace_in_rendered_xml
           assert_match(/xmlns="bar"/, subject.to_xml)
         end
 
+        # https://github.com/sparklemotion/nokogiri/issues/3459
+        def test_namespace_set_on_unparented_node_also_sets_a_definition
+          doc = Nokogiri::XML(%{<root xmlns:foo="http://example.com/foo"></root>})
+          ns = doc.root.namespace_definitions.first
+
+          node = doc.create_element("other")
+          node.namespace = ns
+
+          assert_pattern do
+            node.namespace_definitions => [ { prefix: "foo", href: "http://example.com/foo" } ]
+          end
+        end
+
         # issue 771
         def test_format_noblank
           content = <<~XML

test/xml/test_node_reparenting.rb

@@ -845,6 +845,29 @@ def coerce(data)
             end
           end
         end
+
+        # https://github.com/sparklemotion/nokogiri/issues/3459
+        describe "reparenting with duplicate namespace prefixes" do
+          it "stitches together ok" do
+            doc = Nokogiri::XML(<<~XML)
+              <dnd:adventure xmlns:dnd="http://www.w3.org/dungeons#">
+                <dnd:party xmlns:dnd="http://www.w3.org/dragons#">
+                  <dnd:members>
+                  </dnd:members>
+                </dnd:party>
+              </dnd:adventure>
+            XML
+
+            dungeons_ns = doc.root.namespace_definitions.find { |ns| ns.prefix == "dnd" }
+            parent = doc.xpath("//ns:members", ns: "http://www.w3.org/dragons#").first
+
+            node = doc.create_element("character")
+            node.namespace = dungeons_ns
+            parent.add_child(node)
+
+            assert_includes(doc.to_xml, %{<dnd:character xmlns:dnd="http://www.w3.org/dungeons#"/>})
+          end
+        end
       end
     end
   end