Ensure IPv6 hostname is enclosed within square brackets in Host header
Fixes nodejs/node#5308
As per https://tools.ietf.org/html/rfc7230#section-5.4 and https://tools.ietf.org/html/rfc3986#section-3.2.2 IPv6 addresses in Host header (URI), must be enclosed within square brackets, in order to properly separate the host address from any port reference.