allow table for multiple values in ngx.header['WWW-Authenticate'] fixes #2400

[BotoX]

With this patch it's possible to send multiple WWW-Authenticate headers from Lua:

Like so:

ngx.header["WWW-Authenticate"] = {"Negotiate", "NTLM", "Basic"}

And actually get:

www-authenticate: Negotiate
www-authenticate: NTLM
www-authenticate: Basic

instead of just one of them.

Fixes #2400