Fix dsn.path where Sentry runs at a non-root URL

with a DSN like `https://A:B@host.example.com/sentry/123`, `parseDSN()` ends up like:

```
{ protocol: 'https',
  public_key: 'A',
  private_key: 'B',
  host: 'host.example.com',
  path: 'sentry',
  project_id: 123,
  port: 443 }
```

which means when a request is sent, it goes to `sentry/api/store/` instead of `/sentry/api/store/`.

The change fixes `dsn.path` to be `/sentry` for such URLs, and the existing `''` when sentry is running at the domain root.