[BUG] Fix empty X-Forwarded-For header name when set in defaults section

[BUG] Fix empty X-Forwarded-For header name when set in defaults section

The following patch introduced a minor bug :
   [MINOR] permit renaming of x-forwarded-for header

If "option forwardfor" is declared in a defaults section, the header name
is never set and we see an empty header name before the value. Also, the
header name was not reset between two defaults sections.