Allow to set basePath of the REST API to '/'

**Describe the bug**
Setting `basePath` in cube.js to `/` results in 404 across all API endpoints

**To Reproduce**
Steps to reproduce the behavior:
1. Create a cube.js fils
2. Set `basePath: '/'`
3. Try accessing the API at [base host]/v1/meta
4. Receive a 404

**Expected behavior**
The API should be accessible at [base host]/v1/meta

**Version:**
v0.35.1

**Additional context**
Setting the `basePath` to a different value, e.g. `/test` results in proper behavior ([base host]/test/v1/meta returns 200)