Use "client_header_timeout" for all requests in a connection.

Previously, only the first request in a connection used timeout
value from the "client_header_timeout" directive while reading
header.  All subsequent requests used "keepalive_timeout" for
that.

It happened because timeout of the read event was set to the
value of "keepalive_timeout" in ngx_http_set_keepalive(), but
was not removed when the next request arrived.