fix: isISO31661Alpha2 currently allowing lowercase characters

[jpgorman]

Description

The current IsISO31661Alpha2 method used to check country codes allows for lower case country codes. https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2

Minimal code-snippet showcasing the problem

import {isISO31661Alpha2} from 'classValidator'
expect(isISO31661Alpha2('za')).toBe(true)

Expected behavior

import {isISO31661Alpha2} from 'classValidator'
// passes
expect(isISO31661Alpha2('za')).toBe(false)

Actual behavior

import {isISO31661Alpha2} from 'classValidator'
// fails
expect(isISO31661Alpha2('za')).toBe(false)