fix(jruby): write_to enc falls back to document encoding

This is what libxml2's `xmlsave.c` functions do, so let's make it
explicit in the caller so that JRuby has the same behavior.

CHANGELOG.md

@@ -15,6 +15,7 @@ Nokogiri follows [Semantic Versioning](https://semver.org/), please see the [REA
 ### Fixed
 
 * [JRuby] Serializing an HTML4 document with `#write_to` and specifying no save options will properly emit an HTML document anyway, like libxml2 does. Previously JRuby emitted XML in this situation.
+* [JRuby] Serializing with `#write_to` will fall back to the document encoding when no encoding is specified, like libxml2 does. Previously JRuby emitted UTF-8 in this situation.
 
 
 ### Improved

lib/nokogiri/xml/node.rb

@@ -1347,7 +1347,7 @@ def to_xhtml(options = {})
       #
       def write_to(io, *options)
         options = options.first.is_a?(Hash) ? options.shift : {}
-        encoding = options[:encoding] || options[0]
+        encoding = options[:encoding] || options[0] || document.encoding
         if Nokogiri.jruby?
           save_options = options[:save_with] || options[1]
           indent_times = options[:indent] || 0