8043119: (props) Properties.storeToXML closes output stream

[Alan Bateman]

This is a follow-up to the recent change to the modules motivated switch 
of the Properties XML methods to consistently use the UKit parser. An 
issue that has surfaced since is that the wrapper around UKit is 
incorrectly closing the output stream after writing the XML document, a 
basic assertion not covered by the tests in the jdk repository. Trivial fix:

http://cr.openjdk.java.net/~alanb/8043119/webrev/

-Alan.