[BUGFIX beta] Ensure helpers have a consistent API.

[rwjblue]

Having class based helpers return a different API from "simple" helpers is quite confusing. It makes testing a helper significantly different based on the type of helper, and also makes refactoring from simple to class-based a hazzard.

This change ensures that both types of helper return the same basic interface, and enables us to use output similar to the following regardless of the specific type of helper that was exported:

module('foo | Helper', function(hooks) {
  module('unit tests', function(hooks) {
    setupRenderingTest(hooks);

    test('returns some-value', async function(assert) {
      await render(hbs`{{foo 'input' 'values'}}`);

      assert.equal(this.element.textContent, 'some-value', 'helper works!');
    });
  });

  module('unit tests', function(hooks) {
    setupTest(hooks);

    test('returns some-value', function(assert) {
      let helper = this.owner.factoryFor('helper:foo').create();

      assert.equal(helper.compute(['input', 'values']), 'some-value', 'helper works!');
    });
  });
});