Don't rely on host header being present

Currently server crashes on when requested without Host header(if showDir is true).

```
± % http-server -p 8181
Starting up http-server, serving ./ on port: 8181
Hit CTRL-C to stop the server
```
```
± % echo "GET / HTTP/1.1\n"  | nc 127.0.0.1 8181
```
```
± % http-server -p 8181
/usr/lib/node_modules/http-server/node_modules/ecstatic/node_modules/ent/index.js:12
        throw new TypeError('Expected a String');
              ^
TypeError: Expected a String
    at Object.exports.encode (/usr/lib/node_modules/http-server/node_modules/ecstatic/node_modules/ent/index.js:12:15)
    at module.exports (/usr/lib/node_modules/http-server/node_modules/ecstatic/lib/ecstatic/showdir.js:103:19)
    at module.exports.fs.readdir.sortByIsDirectory (/usr/lib/node_modules/http-server/node_modules/ecstatic/lib/ecstatic/showdir.js:62:17)
    at Object.oncomplete (fs.js:297:15)
```