[FIXED JENKINS-19752] write in the correct outputstream

This fixes the isse that appears in 1.532 and that corrupts all the zip generated by DirectoryBrowserSupport (https://issues.jenkins-ci.org/browse/JENKINS-19752)

http://job/zenika/lastSuccessfulBuild/artifact/repository/output/*zip*/output.zip

The process was not writing in the correct output stream. 
This issue is quite annoying as you cannot download anymore the artifacts in a single file (zip)