Fixes blueprint podPath when podModulePrefix is not defined

When generating a blueprint with `--pod`, the podPath was resolving to `/`, which was adding an extra slash to the path. For instance, without `podModulePrefix`, the following would happen:
```
ember g route foo -p
version: 0.0.46
installing
  create app//foo/route.js
  create app//foo/template.hbs
installing
  create tests/unit/routes/foo-test.js
```

This has been fixed, and additional tests to verify pod generation with and without `podModulePrefix` have been added.