Update zlib from 1.2.11 to 1.2.12

fix #2439

Because there is a CVE issue impact zlib before 1.2.12,
update zlib to avoid any related problems.

Please check the link below for detailed CVE information:
https://access.redhat.com/security/cve/CVE-2018-25032

test/beast/zlib/zlib.cpp

@@ -8,5 +8,5 @@
 //
 
 // Make sure symbols don't conflict with ZLib
-#include "zlib-1.2.11/zlib.h"
+#include "zlib-1.2.12/zlib.h"
 #include <boost/beast/zlib.hpp>