Upcase byDay even when used with bySetPos

[rubys]

Standalone test case:

const icaljs = require('ical.js');
const icalgen = require('ical-generator');

let event = {
  uid: 'unique',
  summary: 'repeat every third sunday',
  start: '2020-02-20T12:00:00Z',
  repeating: {
    freq: 'MONTHLY', // required
    count: 5,
    byDay: 'su', // repeat only sunday
    bySetPos: 3, // repeat every 3rd sunday
  }
}

let ical = icalgen({});
ical.createEvent(event);
console.log(ical.toString());
icaljs.parse(ical.toString());

[sebbo2002]

Just published version 1.12.1, thanks for your PR ❤️

[sebbo2002]

🎉 This PR is included in version 1.3.0-develop.1 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release
• v1.3.0-develop.1

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