Parsed URL objects have a colon in protocol field.

When I try to parse a url using `url.parse()`, the protocol has a colon at the end. 
### Example

The following command

```
url.parse("https://avatars3.githubusercontent.com/u/2100222")
```

returns

```
Url {
  protocol: 'https:',
  ...
}
```

Shouldn't the protocol be `http` instead of `http:`? If not, what's the point of having it included?
