Fix FasterXML#262

# Conflicts:
#	release-notes/CREDITS-2.x
#	release-notes/VERSION-2.x

Author: cowtowncoder

src/main/java/com/fasterxml/jackson/dataformat/xml/ser/ToXmlGenerator.java

@@ -27,7 +27,8 @@
  * additional configuration calls beyond regular {@link JsonGenerator} API,
  * mostly to pass namespace information.
  */
-public final class ToXmlGenerator
+public class ToXmlGenerator
+// non-final since 2.12 but only sub-class if you really know what you are doing...
     extends GeneratorBase
 {
     /**