fixes sparklemotion#775.

libxml will throw an error if we tried to replace a text node
in a document fragment since it tries to find an element node
or a document node in the parents of the node being replaced.
To make this possible we add an element node and replace that
node instead.

lib/nokogiri/xml/node.rb

@@ -376,17 +376,22 @@ def children= node_or_tags
       #
       # Also see related method +swap+.
       def replace node_or_tags
+        # We cannot replace a text node directly, otherwise libxml will return
+        # an internal error at parser.c:13031, I don't know exactly why
+        # libxml is trying to find a parent node that is an element or document
+        # so I can't tell if this is bug in libxml or not. issue #775.
+        if text?
+          replacee = Nokogiri::XML::Node.new 'dummy', document
+          add_previous_sibling_node replacee
+          unlink
+          return replacee.replace node_or_tags
+        end
+
         node_or_tags = coerce(node_or_tags)
+
         if node_or_tags.is_a?(XML::NodeSet)
-          if text?
-            replacee = Nokogiri::XML::Node.new 'dummy', document
-            add_previous_sibling_node replacee
-            unlink
-          else
-            replacee = self
-          end
-          node_or_tags.each { |n| replacee.add_previous_sibling n }
-          replacee.unlink
+          node_or_tags.each { |n| add_previous_sibling n }
+          unlink
         else
           replace_node node_or_tags
         end

test/xml/test_document_fragment.rb

@@ -8,6 +8,13 @@ def setup
         @xml = Nokogiri::XML.parse(File.read(XML_FILE), XML_FILE)
       end
 
+      def test_replace_text_node
+        html = "foo"
+        doc = Nokogiri::XML::DocumentFragment.parse(html)
+        doc.children[0].replace "bar"
+        assert_equal 'bar', doc.children[0].content
+      end
+
       def test_fragment_is_relative
         doc      = Nokogiri::XML('<root><a xmlns="blah" /></root>')
         ctx      = doc.root.child