Triple slashes

Although it is okay, still looks weird:

```
>>> f = furl('staging.s3-website.us-east-1.amazonaws.com')
>>> f.scheme = 'http'
>>> f.url
'http:///staging.s3-website.us-east-1.amazonaws.com'
```

Why is there triple slashes?

Additional:
```
>>> f = furl('http://staging.s3-website.us-east-1.amazonaws.com')
>>> f.url
'http://staging.s3-website.us-east-1.amazonaws.com'
```

But:
```
>>> f = furl('staging.s3-website.us-east-1.amazonaws.com')
>>> f.scheme = 'http://' # yes, it's not a proper way, but still confusing how parser works
'http://:staging.s3-website.us-east-1.amazonaws.com'
```

I'm confused.