Fix XML::DocumentFragment to return an instance of callee's class

Fixes #1846. Bug introduced in f2f3a3b.

CHANGELOG.md

@@ -1,3 +1,10 @@
+# 1.9.1 / 2018-12-17
+
+## Bug fixes
+
+* Fix a bug introduced in v1.9.0 where `XML::DocumentFragment#dup` no longer returned an instance of the callee's class, instead always returning an `XML::DocumentFragment`. This notably broke any subclass of `XML::DocumentFragment` including `HTML::DocumentFragment` as well as the Loofah gem's `Loofah::HTML::DocumentFragment`. [#1846]
+
+
 # 1.9.0 / 2018-12-17
 
 ## Security Notes

lib/nokogiri/xml/document_fragment.rb

@@ -28,7 +28,7 @@ def initialize document, tags = nil, ctx = nil
       if Nokogiri.uses_libxml?
         def dup
           new_document = document.dup
-          new_fragment = XML::DocumentFragment.new(new_document)
+          new_fragment = self.class.new(new_document)
           children.each do |child|
             child.dup(1, new_document).parent = new_fragment
           end

test/html/test_document_fragment.rb

@@ -305,6 +305,13 @@ def test_capturing_nonparse_errors_during_node_copy_between_fragments
         assert_equal original_errors1, frag1.errors
         assert_equal original_errors2, frag2.errors
       end
+
+      def test_dup_should_create_an_html_document_fragment
+        # https://github.com/sparklemotion/nokogiri/issues/1846
+        original = Nokogiri::HTML::DocumentFragment.parse("<div><p>hello</p></div>")
+        duplicate = original.dup
+        assert_instance_of Nokogiri::HTML::DocumentFragment, duplicate
+      end
     end
   end
 end

test/xml/test_document_fragment.rb

@@ -263,6 +263,20 @@ def test_issue_1077_parsing_of_frozen_strings
         Nokogiri::XML::DocumentFragment.parse(input) # assert_nothing_raised
       end
 
+      def test_dup_should_exist_in_a_new_document
+        # https://github.com/sparklemotion/nokogiri/issues/1063
+        original = Nokogiri::XML::DocumentFragment.parse("<div><p>hello</p></div>")
+        duplicate = original.dup
+        assert_not_equal original.document, duplicate.document
+      end
+
+      def test_dup_should_create_an_xml_document_fragment
+        # https://github.com/sparklemotion/nokogiri/issues/1846
+        original = Nokogiri::XML::DocumentFragment.parse("<div><p>hello</p></div>")
+        duplicate = original.dup
+        assert_instance_of Nokogiri::XML::DocumentFragment, duplicate
+      end
+
       def test_dup_creates_tree_with_identical_structure
         original = Nokogiri::XML::DocumentFragment.parse("<div><p>hello</p></div>")
         duplicate = original.dup