BUG/MINOR: lua: set current proxy as default value if it is possible

BUG/MINOR: lua: set current proxy as default value if it is possible

Some fetches uses a proxy name as first parameter. This is used to identify
frontend, backend or table. This first argument is declared as mandatory,
but the documentation says that is optional.

The behavior of the function smp_resolve_args() is to use the current proxy
if it match the required argument.

This patch implements the same behavior in the Lua argument checker for
sample-fetches and sample-converters.