fix(docs): Fix docs and test about email validation - following API R…

[klausetgeton]

…EST v4 specs

Hello, this PR just intends to update the docs due to the different response returned from the API REST v4 which is used by the library to individually validate the email address.

• The response added as an example in this PR follows the standard from the Mailgun API REST v4 - https://documentation.mailgun.com/en/latest/api-email-validation.html?highlight=validate#request-examples
• As you can see it should be something like

{
    "address": "existingemail@realdomain.com",
    "is_disposable_address": false,
    "is_role_address": false,
    "reason": [],
    "result": "deliverable",
    "risk": "low"
}

• The response format in the test and in the docs are following the deprecated v3 response format, even though the tests are clear that the API v4 is used under the hood for validating the email. API REST v3 (Deprectated) https://documentation.mailgun.com/en/latest/api-email-validation-deprecated.html?highlight=is_valid#example

{
    "address": "foo@mailgun.net",
    "did_you_mean": null,
    "is_disposable_address": false,
    "is_role_address": true,
    "is_valid": true,
    "mailbox_verification": "true",
    "parts": {
        "display_name": null,
        "domain": "mailgun.net",
        "local_part": "foo"
    }
}

• The docs on NPM are not following the actual response as well - https://www.npmjs.com/package/mailgun.js#get-5

Please let me know if there is something I could do better and clarify.

Thanks in advance!

Best regards, Klaus.