Default server host should be localhost

[pinheadmz]

The main reason for this is to match the default in bcurl/client:

https://github.com/bcoin-org/bcurl/blob/d7e088fad4c284fb5d6fd7205c6b903bd3e6bf83/lib/client.js#L371

Right now bweb hosts explicitly on ipv4:

bweb/lib/server.js

Line 679 in 98327b4

this.host = '127.0.0.1';

and I believe this is causing issues on ipv6 enabled machines, such as handshake-org/hs-client#45

[pinheadmz]

TODO follow-up, do we need to add "localhost" to this list of auto-authorized hostnames?

https://github.com/handshake-org/hsd/blob/master/lib/node/http.js#L890-L895

[pinheadmz]

closing, see #12