Fetch: fixed compatibility with nginx-1.22 and below.

Fetch: fixed compatibility with nginx-1.22 and below.

Previously, ngx_table_elt_t structure was used directly in njs fetch
code.  This prevents from building with nginx-1.22 as the structure
was changed after nginx-1.22.

The fix is to use njs defined structure instead of ngx_table_elt_t.

This fixes #610 issue on Github.