add ability to handle equality rule by validate.js

[drselump14]

Equality rule on validate.js requires the value of another attribute.
This PR adding ability to handle the equality rule.

[jwald1]

Thanks @drselump14 for the PR. Line 55 and 62 needs to be changed to let

// changed some var names
validatejsParams(attributeName) {
    const rule = this.rules[attributeName]

    if (!rule) {
      return
    }
   // const won't work
    let { value } = this.attributes.get(attributeName)
    const attributes = {
      [attributeName]: value,
    }

    if (rule.equality) {
      const otherAttributeName = rule.equality.attribute
      // const won't work
      let { value } = this.attributes.get(otherAttributeName)
      attributes[otherAttributeName] = value
    }

    return [attributes, { [attributeName]: this.rules[attributeName] }]
  }

Also, I think that it should be added to the README that equality validation needs to be set in both fields. For example, password and password_confirmation, if it's only set on the password_confirmation field than the user can change the password field without getting an error.

[drselump14]

Hello, thanks for your comment.

I've updated the readme for the explanation of equality password validation.

Regarding your advice:

1. I think it's better to use const instead of let in this context. I don't see any necessity to reassign the value, so const is fine. And if we compare it with previous implementation, most of them are defined in const. So, I don't think let is suitable in this case.

2. The issue of putting an equality rule on password is, it will validate the password field before password_confirmation field has a value which in my opinion is not good approach from UX perspective.