Fix bug from r36972 that caused some test failures.. that change wrote

PRE/POSTAMBLE to Base64 encoded stream, so no preambles are found when
the console bytes are scanned (as they got encoded).
Fix this, and also apply the atomic-write change to encodeTo(Writer).


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@37064 71c3de6d-444a-0410-be80-ed276b4c234a