Detect old-style S3 URL for auto-sigv4.

This fixes detection of signature version 4 for the eu-central-1 region
when creating connections using the old-style URL, which is listed on
the [supported regions and endpoints](http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region):

```
s3-eu-central-1.amazonaws.com
```

cc @kyleknap, @jamesls