fix(persona): reference model using getModel method

this._model is null initially, so instead make use of this.getModel()

Author: thetutlage

src/Persona.js

@@ -483,7 +483,7 @@ class Persona {
       await callback(payload)
     }
 
-    const user = await this._model.create(payload)
+    const user = await this.getModel().create(payload)
 
     /**
      * Get email verification token for the user

test/persona.spec.js

@@ -735,4 +735,18 @@ test.group('Persona', (group) => {
     const token1 = await this.persona.generateToken(user1, 'email')
     assert.notEqual(token, token1)
   })
+
+  test('use custom registration rules', async (assert) => {
+    this.persona.registerationRules = function () {
+      return {
+        username: 'required'
+      }
+    }
+
+    await this.persona.register({ username: 'virk', email: '[email protected]' })
+    const users = await getUser().all()
+
+    assert.equal(users.size(), 1)
+    assert.equal(users.first().email, '[email protected]')
+  })
 })