BUG/MEDIUM: hlua: Use front SC to detect EOI in HTTP applets' receive functions

BUG/MEDIUM: hlua: Use front SC to detect EOI in HTTP applets' receive functions

When an HTTP applet tries to get request data, we must take care to properly
detect the end of the message. It an empty HTX message with the SC_FL_EOI
flag set on the front SC. However, an issue was introduced during the SC
refactoring performed in the 2.8. The backend SC is tested instead of the
frontend one.

Because of this bug, the receive functions hang because the test on
SC_FL_EOI flag never succeeds. Of course, by checking the frontend SC (the
opposite SC to the one attached to the appctx), it works.

This patch should fix the issue #2180. It must be backported to the 2.8.