Always use XPathFunctions

ext/java/nokogiri/XmlXpathContext.java

@@ -106,7 +106,7 @@ public class XmlXpathContext extends RubyObject
   {
     String query = rbQuery.convertToString().asJavaString();
 
-    if (!handler.isNil() && !isContainsPrefix(query)) {
+    if (!isContainsPrefix(query)) {
       //
       //  The user has passed in a handler, but isn't using the `nokogiri:` prefix as
       //  instructed in JRuby land, so let's try to be clever and rewrite the query, inserting
@@ -131,9 +131,10 @@ public class XmlXpathContext extends RubyObject
         jchar = xpathFunctionCalls.end();
       }
 
-      if (jchar < query.length() - 1) {
+      if (jchar < query.length()) {
         namespacedQuery.append(query.subSequence(jchar, query.length()));
       }
+
       query = namespacedQuery.toString();
     }
 

lib/nokogiri/xml/searchable.rb

@@ -253,7 +253,7 @@ def extract_params(params) # :nodoc:
           ![Hash, String, Symbol].include?(param.class)
         end
         params -= [handler] if handler
-        handler = XPathFunctions.new(handler) if handler
+        handler = XPathFunctions.wrap(handler)
 
         hashes = []
         while Hash === params.last || params.last.nil?

lib/nokogiri/xml/xpath_functions.rb

@@ -5,6 +5,15 @@
 module Nokogiri
   module XML
     class XPathFunctions < SimpleDelegator
+      class << self
+        def wrap(handler)
+          if handler.nil?
+            @wrap_nil ||= new(Object.new)
+          else
+            new(handler)
+          end
+        end
+      end
     end
   end
 end