filter/xml fix for LOGSTASH-2246: extract non-ascii content with xpath

As reported in [LOGSTASH-2246](https://logstash.jira.com/browse/LOGSTASH-2246), xml filter xpath fails with non-ascii content,
here is a test case and fix.

It is due to Nokogiri [to_s](http://nokogiri.org/Nokogiri/XML/Node.html#method-i-to_s) calling to_xhml or to_xml without encoding parameters.
using the to_str method extract the content differently (hidden in the C or Java implementation) and do not suffer encoding issue