Fix a bug that XPath can't be used for no document element (#268)

Fixes #267 

#249 improved performance by introducing cache. It requires document but
we should not break backward compatibility for performance improvement.

This restores the previous behavior but no document case doesn't have
performance improvement introduced by #249.

Author: pboling

lib/rexml/xpath_parser.rb

@@ -84,7 +84,12 @@ def parse path, node
         node = node.first
       end
 
-      node.document.__send__(:enable_cache) do
+      document = node.document
+      if document
+        document.__send__(:enable_cache) do
+          match( path_stack, node )
+        end
+      else
         match( path_stack, node )
       end
     end

test/parser/test_xpath.rb

@@ -4,7 +4,7 @@
 require "rexml/parsers/xpathparser"
 
 module REXMLTests
-  class TestXPathParser < Test::Unit::TestCase
+  class TestParserXPathParser < Test::Unit::TestCase
     sub_test_case("#abbreviate") do
       def abbreviate(xpath)
         parser = REXML::Parsers::XPathParser.new

test/test_xpath_parser.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module REXMLTests
+  class TestXPathParser < Test::Unit::TestCase
+    def setup
+      @root_element = make_service_element(["urn:type1", "urn:type2"], ["http://uri"])
+      @element = @root_element.children[0]
+      @parser = REXML::XPathParser.new
+    end
+
+    def make_service_element(types, uris)
+      root_element = REXML::Element.new
+      element = root_element.add_element("Service")
+      types.each do |type_text|
+        element.add_element("Type").text = type_text
+      end
+      uris.each do |uri_text|
+        element.add_element("URI").text = uri_text
+      end
+      root_element
+    end
+
+    def test_found
+      res = @parser.parse("/Service", @root_element)
+      assert_equal([@element],
+                   res)
+    end
+
+    def test_not_found
+      res = @parser.parse("/nonexistent", @root_element)
+      assert_equal([],
+                   res)
+    end
+  end
+end