It doesn't work with jspm.dev

It seems that when an import doesn't include the host, it fall backs to https://unpkg.com, but it causes jpm.dev modules to fail.

for example, if you have the following:
```
import 'https://jspm.dev/bootstrap-confirmation2@4.1.0';
```

jspm.dev returns the following:
```

import "/npm:jquery@3!cjs";
import "/npm:bootstrap!cjs";
export { default } from "/npm:bootstrap-confirmation2@4.1.0!cjs";

```

It works on the browser. It downloads https://jspm.dev/npm:jquery@3!cjs. But import-http plugin tries to fetch https://unpkg.com//npm:jquery@3!cjs which fails.