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`.
We don't currently have a regular expression that does this. We have 3 options as I see them:

1. continue to rely on `urlparse` from the standard library
1. use `urllib3`'s URL parser
1. use `rfc3986`'s URI parser

They all, to varying degrees, follow the specification and will have very similar, if not exactly the same behaviour. We are far more likely to rely on third party libraries that do things efficiently to regular expressions that we put together ourselves.