Fix slow response time of requests

[eddami]

I've noticed that the response times of requests going through this middleware are slow (2s-4s in my case).
After a bit of investigation, i found this discussion about UseProxyToSpaDevelopmentServer which is used internally in this library.
It turns out that webpack/nodejs server is listening to IPv4 only. However, SpaProxy on localhost tries IPv6 first so it takes few seconds before trying IPv4.
The solution is to target 127.0.0.1 directly.

This is some timing analysis:
With localhost:

With 127.0.0.1:

[EEParker]

Thank you for the PR!