Use UTF-8 encoding when generating XML

[olieidel]

Hey, first off, thanks so much for your work on this library, it's really cool!

While playing around with it, I noticed that the formatting of some German characters (ß, ö, ä, ü) was screwed up. I then compared the xml output of this library with the factur-x.xml output of a third-party tool (easybill). I noticed that the first line of the xml differs:

ruby-secretariat:

<?xml version="1.0"?>
<!-- ... -->

Third-party tool (easybill):

<?xml version="1.0" encoding="UTF-8"?>
<!-- ... -->

So the conclusion here is that the encoding is set to UTF-8 explicitly and that that fixes the weird formatting of the characters.

Next, I checked how to add this to ruby-secretariat, and the change is simply to pass the UTF-8 encoding to Nokogiri when instantiating it - check out the diff to see what I mean.

This fixes the problem. The XML output now contains the encoding="UTF-8", and the characters are formatted correctly.