MINOR: fcgi-app: Stop on last HTX block when getting the payload size

MINOR: fcgi-app: Stop on last HTX block when getting the payload size

In the loop counting all DATA block to be able to add a "Content-length:"
header in the FCGI response, we now check flags on the HTX block to know if
it is the last one of the message or not. If HTX_BLK_FL_EOM flag is set, it
means we can stop.