SPDY: fix support for headers with multiple values.

Split SPDY header with multiple, NULL-separated values:

    cookie: foo\0bar

into two separate HTTP headers with the same name:

    cookie: foo
    cookie: bar

Even though the logic for this behavior already existed
in the source code, it doesn't look that it ever worked
and SPDY streams with such headers were simply rejected.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>