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`.
@sigmavirus24 
I would argue that this is an implementation detail, and the test should be independent of this, and it's better to have more tests than less.
It might happen that the test for skipping wouldn't catch IP addresses, but only domains.

For example, given an IP address and/or the dots in the "scheme", it would be possible to not use this as a scheme. These tests are meant to keep this behavior.