Added support for nginx proxy_set_header X-Request-Start $msec and t=${msec}

Thanks for the awesome gem!  This commit makes it easier to configure `X-Request-Start` for Nginx.  It adds support for the following configurations:

```conf
proxy_set_header X-Request-Start $msec;
```

or

```conf
proxy_set_header X-Request-Start "t=${msec}";
```