Automatically detect the fastest XML parser

This should use libxml2 if available, and fall back to rexml otherwise.
Recent version of rexml have a tag limit to mitigate some DoS attacks,
which can easily be reached with the verbosity of XML-RPC.

lib/xmlrpc/config.rb

@@ -15,7 +15,11 @@ module Config
     #
     # * XMLParser::REXMLStreamParser
     # * XMLParser::LibXMLStreamParser
-    DEFAULT_PARSER = XMLParser::REXMLStreamParser
+    begin
+      DEFAULT_PARSER = XMLParser::LibXMLStreamParser.new.class
+    rescue LoadError
+      DEFAULT_PARSER = XMLParser::REXMLStreamParser.new.class
+    end
 
     # enable <code><nil/></code> tag
     ENABLE_NIL_CREATE    = false