BUG/MEDIUM: http: fix body processing for the stats applet

BUG/MEDIUM: http: fix body processing for the stats applet

Commit 9fbe18e ("MEDIUM: http: add a new option http-buffer-request")
introduced a regression due to a misplaced check causing the admin
mode of the HTTP stats not to work anymore.

This patch tried to ensure that when we need a request body for the
stats applet, and we have already waited for this body, we don't wait
for it again, but the condition was applied too early causing a
disabling of the entire processing the body, and based on the wrong
HTTP state (MSG_BODY) resulting in the test never matching.

Thanks to Chad Lavoie for reporting the problem.

This bug is 1.6-only, no backport is needed.