RFR (JAXP) 8139584: XMLStreamWriterImpl does not write 'standalone' property

[Joe Wang]

Hi,

Please review a fix for declaring the 'standalone' attribute.

JBS: https://bugs.openjdk.java.net/browse/JDK-8139584
webrev: http://cr.openjdk.java.net/~joehw/jdk9/8139584/webrev/

Thanks,
Joe