Merge of r4530, r4531: null character fixes.

Merge of r4530, r4531: null character fixes.

*) Fixed incorrect ngx_cpystrn() usage in ngx_http_*_process_header().

   This resulted in a disclosure of previously freed memory if upstream
   server returned specially crafted response, potentially exposing
   sensitive information.

   Reported by Matthew Daley.

*) Headers with null character are now rejected.

   Headers with NUL character aren't allowed by HTTP standard and may cause
   various security problems.  They are now unconditionally rejected.