MINOR: stats admin: allow unordered parameters in POST requests

MINOR: stats admin: allow unordered parameters in POST requests

Previously, the stats admin page required POST parameters to be provided
exactly in the same order as the HTML form.
This patch allows to handle those parameters in any orders.

Also, note that haproxy won't alter server states anymore if backend or server
names are ambiguous (duplicated names in the configuration) to prevent
unexpected results (the same should probably be applied to the stats socket).