The PRESERVE_ASCII_NULL option prevents WriteAscii from converting
\0 to white space.

R=verwaest@chromium.org
BUG=v8:2252
TEST=test-api/StringWrite

Review URL: https://chromiumcodereview.appspot.com/10810056

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12174 ce2b1a6d-e550-0410-aec6-3dcde31c8c00