Use MultiXML constant to fix deprecation warning from multi_xml 0.9+

Falls back to MultiXml for older versions so no minimum version bump
is required.

Author: robzolkos

lib/oauth2/response.rb

@@ -171,7 +171,7 @@ def parser
 OAuth2::Response.register_parser(:xml, ["text/xml", "application/rss+xml", "application/rdf+xml", "application/atom+xml", "application/xml"]) do |body|
   next body unless body.respond_to?(:to_str)
 
-  MultiXml.parse(body)
+  (defined?(MultiXML) ? MultiXML : MultiXml).parse(body)
 end
 
 # Register JSON parser

spec/config/multi_xml.rb

@@ -2,4 +2,4 @@
 
 require "multi_xml"
 
-MultiXml.parser = :rexml
+(defined?(MultiXML) ? MultiXML : MultiXml).parser = :rexml

spec/oauth2/client_spec.rb

@@ -767,7 +767,8 @@
           end
         end
 
-        expect { client.get_token(parse: :xml) }.to raise_error(MultiXml::ParseError)
+        xml_parse_error = defined?(MultiXML) ? MultiXML::ParseError : MultiXml::ParseError
+        expect { client.get_token(parse: :xml) }.to raise_error(xml_parse_error)
       end
     end