RFR(jdk 13/java.xml) 8219705: Wrong media-type for a given serialization method

[Joe Wang]

Please review a fix for a JCK failure. All XML-related JCK and SQE tests 
passed after the patch.

JBS: https://bugs.openjdk.java.net/browse/JDK-8219705
webrevs: http://cr.openjdk.java.net/~joehw/jdk13/8219705/webrev/

Thanks,
Joe