WL#2386: Use correct character set in mysql and mysqldump --xml output

Currently, the mysql and mysqldump command-line clients have --xml flags to enable 
XML output that is not character-set aware. Either they should always output UTF-8 
output (easiest), or the character encoding of the output should be set in the XML 
prologue like: 

Always forcing UTF-8 has the advantage that the output will then always be 
understood by all XML parsers (since UTF-8 is a basic requirement of XML parsers), 
but other encoding types may not be.

this is a duplicate of WL#1587