Proxy: fixed possible uninitialized memory access.

Proxy: fixed possible uninitialized memory access.

The ngx_http_proxy_rewrite_cookie() function expects the value of the
"Set-Cookie" header to be null-terminated, and for headers obtained
from proxied server it is usually true.

Now the ngx_http_proxy_rewrite() function preserves the null character
while rewriting headers.

This fixes accessing memory outside of rewritten value if both the
"proxy_cookie_path" and "proxy_cookie_domain" directives are used in
the same location.