Authorization header is sent even without --username and --password

```sh
rawler https://nativescript.org/
```
 fails because the request looks like this:

```
GET https://www.nativescript.org/ HTTP/1.1
Accept-Encoding: gzip;q=1.0,deflate;q=0.6,identity;q=0.3
Accept: */*
User-Agent: Ruby
Authorization: Basic Og==
Connection: close
Host: www.nativescript.org
```

The server returns HTTP/500 only because of `Authorization: Basic Og==`.