[0.5.x] Re-work on-change validation

[timacdonald]

Precognition does its best to avoid sending requests when it does not need to. This might happen when the validation function has been called but the form's has not changed since the last validation attempt.

You could imagine a text input where the user changes a value and before leaving the field restores the original value. In that scenario there is no need to send a validation request as the form has not changed and the existing validation errors etc still makes sense.

This is the expected behaviour when you are validating a single field and the user is implicitly invoking the validate function.

<input
   v-model="form.email"
   @change="form.validate('name')"
>

However, when building a wizard and calling form.validate() in a button without specifying a particular input to validate, you expect that the validation process will occur regardless of the state of the form so you are able to react to the response.

<button
    type="button"
    @click="form.touch(['name', 'email', 'phone']).validate({
        only: ['name', 'email', 'phone'],
        onSuccess: (response) => nextStep(),
        onValidationError: (response) => /* ... */,
    })"
>Next Step</button>

The changes in this PR ensure the second scenario will send a validation request even when the form has not changed since the last validation attempt.

[timacdonald]

Small type improvement.

[timacdonald]

Small type improvement.