Hello! Great project.

To jump right in, it is a hacky experience to create an axios client for a single-endpoint API that also uses a file extension. ie `https://coolapi.com/api.php`

All url requests make their way through `combineURLs.js`, to be appended with a `/` regardless of if there was a relative url attached to the client's request config.

The current hack (it feels like hack...) to get around this is to add a request interceptor to remove the ending slash. I'm currently doing this in a project.

Happy to PR this issue with a fix to combineURLs - but I want to run it past owners first for validity.

Cheers! ✨ 
test/specs/helpers/combineURLs.spec.js
var combineURLs = require('../../../lib/helpers/combineURLs');
describe('helpers::combineURLs', function () {
  it('should combine URLs', function () {
    expect(combineURLs('https://api.github.com', '/users')).toBe('https://api.github.com/users');
  });
  it('should remove duplicate slashes', function () {
    expect(combineURLs('https://api.github.com/', '/users')).toBe('https://api.github.com/users');
  });
  it('should insert missing slash', function () {
    expect(combineURLs('https://api.github.com', 'users')).toBe('https://api.github.com/users');
  });
});