Allow reparenting nodes to be a child of an empty document.

Fixes #1773

Author: jvshahid

ext/java/nokogiri/XmlNode.java

@@ -1542,6 +1542,10 @@ protected IRubyObject adoptAs(ThreadContext context, AdoptScheme scheme,
          try {
             Document prev = otherNode.getOwnerDocument();
             Document doc = thisNode.getOwnerDocument();
+            if (doc == null && thisNode instanceof Document) {
+              // we are adding the new node to a new empty document
+              doc = (Document) thisNode;
+            }
             clearXpathContext(prev);
             clearXpathContext(doc);
             if (doc != null && doc != otherNode.getOwnerDocument()) {

test/xml/test_node_reparenting.rb

@@ -197,6 +197,17 @@ class TestNodeReparenting < Nokogiri::TestCase
             end
           end
 
+          describe "given the new document is empty" do
+            it "adds the node to the new document" do
+              doc1 = Nokogiri::XML.parse("<value>3</value>")
+              doc2 = Nokogiri::XML::Document.new
+              node = doc1.at_xpath("//value")
+              node.remove
+              doc2.add_child(node)
+              assert_match /<value>3<\/value>/, doc2.to_xml
+            end
+          end
+
           describe "given a parent node with a default namespace" do
             before do
               @doc = Nokogiri::XML(<<-eoxml)