Add tests to test_invalid_url for `InvalidSchema`

In b149be5d `PreparedRequest` was made to skip `parse_url` for e.g.
`$HOST:$PORT`, which results in `MissingSchema` not being raised for
`localhost:3128`.
I am referring to this code (https://github.com/kennethreitz/requests/blob/master/requests/models.py#L337-L342):

        if ':' in url and not url.lower().startswith('http'):
            self.url = url
            return

If this would get changed, an IP address might get handled different from a hostname.

Here btw `urllib3`s URL parser is being used further down.