Switched from isomorphic-fetch to cross-fetch and added tests

This changes the internals to cross-fetch, which allows the use of blobs. isomorphic fetch did not support this so we have now moved over to cross-fetch.

In addition to that, this packages also has a lot of users now, so I've added a test suite to try and ensure that it doesn't break with any additional changes. Moving forward I will only allow PRs that write some additional tests to ensure additional functionality is work correctly and also doesn't break current tests.