CVE-2017-17742: HTTP response splitting in WEBrick

CVE-2018-6914: Unintentional file and directory creation with directory
               traversal in tempfile and tmpdir

CVE-2018-8777: DoS by large request in WEBrick

CVE-2018-8778: Buffer under-read in String#unpack

CVE-2018-8779: Unintentional socket creation by poisoned NUL byte in
               UNIXServer and UNIXSocket

CVE-2018-8780: Unintentional directory traversal by poisoned NUL byte in
               Dir

fixes #8747