Pass through entire path when proxying

When using the `http-proxy` blueprint, a proxied request does not pass through the full path. e.g.

```
ember g http-proxy api http://localhost:5000
ember serve

http://localhost:4200/api/foo - [proxied] -> http://localhost:5000/foo # unexpected

http://localhost:4200/api/foo - [proxied] -> http://localhost:5000/api/foo # expected
```

It looks like it is not going to be supported in `node-http-proxy` since they removed "proxy table" support: https://github.com/nodejitsu/node-http-proxy/issues/386

The fix is simple.