BUG/MEDIUM: hlua: Check the calling direction in lua functions of the HTTP class

BUG/MEDIUM: hlua: Check the calling direction in lua functions of the HTTP class

It is invalid to manipulate responses from http-request rules or to manipulate
requests from http-response rules. When http-request rules are evaluated, the
connection to server is not yet established, so there is no response at all. And
when http-response rules are evaluated, the request has already been sent to the
server.

Now, the calling direction is checked. So functions "txn.http:req_*" can now
only be called from http-request rules and the functions "txn.http:res_*" can
only be called from http-response rules.

This issue was reported on Github (#190).

This patch must be backported to all versions since the 1.6.