Set the string encoding when parsing XML

Improves upon #1790

Fixes #1803

Author: jordansissel

lib/logstash/filters/xml.rb

@@ -91,7 +91,7 @@ def filter(event)
 
     if @xpath
       begin
-        doc = Nokogiri::XML(value)
+        doc = Nokogiri::XML(value, nil, value.encoding.to_s)
       rescue => e
         event.tag("_xmlparsefailure")
         @logger.warn("Trouble parsing xml", :source => @source, :value => value,
@@ -115,7 +115,7 @@ def filter(event)
           unless value.nil?
             matched = true
             event[xpath_dest] ||= []
-            event[xpath_dest] << value.to_str
+            event[xpath_dest] << value.to_s
           end
         end # XPath.each
       end # @xpath.each