Form submission should be prevented of onBeforeAdd prevented chip creation

[theKashey]

Scenario

• you enter for example 123
• you have onBeforeAdd to accept only 6 characters
• it returns false
• chip is not created
• form is submitted

I am referring to this piece of code:
https://github.com/TeamWertarbyte/material-ui-chip-input/blob/master/src/ChipInput.js#L391-L394

should be

 handleAddChip (chip) {
    if (this.props.onBeforeAdd && !this.props.onBeforeAdd(chip)) {
      this._preventChipCreation = true
      return chip ? true : false; // -> allow default only if there is no chip to add
    }

[leMaik]

Good suggestion, PR welcome @theKashey :)

[ghost]

Hi guys, is it ok if I open this PR since it's inactive since September 2020? I'm really needing this fix. Thank you.