haproxy: bump to version 1.5.2

 - [RELEASE] Released version 1.5.2
 - [PATCH 1/2] DOC: mention that Squid correctly responds 400 to PPv2
 - [PATCH 2/2] DOC: fix typo in Unix Socket commands

Signed-off-by: Thomas Heil <heil@terminal-consulting.de>