New Feature: robot_mitigation_hash_input

Now it's possible to set source hash element by using a new directive
'robot_mitigation_hash_input', default value is "$remote_addr".

e.g.

robot_mitigation_hash_input "$remote_addr$uri";

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>
Bugfix: coredump in proxy module

if an upstream block is placed after a proxy_pass direcitve, a coredump
will happen.

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>
Merge pull request #34 from InfoHunter/master

robot mitigation hash input & other bug fixes