NumericField setScale() does not update decimals

Affected Version 4.5.1

Description

When creating a new NumericField in a custom Form and setting a float value to the field, the decimals will get stripped off even if the NumericField::setScale() method is called right after creating the field (before rendering it). This is because the field's construction process casts the value as an integer.

Steps to Reproduce

<?php

use SilverStripe\Control\Controller;
use SilverStripe\Forms\FieldList;
use SilverStripe\Forms\Form;
use SilverStripe\Forms\NumericField;

class SomeController extends Controller
{
    function SomeForm()
    {
        $fields = new FieldList(
            $numeric_field = new NumericField('SomeField', 'Some field', 1.5) // The value 1.5 will be casted to an integer 1
        );
        $numeric_field->setScale(2);
        return new Form($this, 'SomeForm', $fields);
    }
}

The scale is 0 during the construction. The field constructor calls NumericField::setValue(), which performs casting: https://github.com/silverstripe/silverstripe-framework/blob/3ad4b93daacf3dc3d76a3da6c0287827604a631a/src/Forms/NumericField.php#L142-L147 (Note $this->originalValue, I'll come to that soon.)

NumericField::setScale() makes the field float/integer, but does not correct the decimals of the current value: https://github.com/silverstripe/silverstripe-framework/blob/3ad4b93daacf3dc3d76a3da6c0287827604a631a/src/Forms/NumericField.php#L307-L318

So I suggest that we:

To recast the value, add $this->setValue($this->originalValue) to NumericField::setScale() after the $this->scale = $scale; line.
Create a new unit test that ensures that decimals are present correctly after calling setScale(*anything greater than 0*).
Another unit test would ensure that decimals will get dropped off if someone calls setScale(0).

A secondary solution would be to add a $scale parameter to NumericField::__construct() and set the scale before setting the value. Or maybe do both of these?

silverstripe / silverstripe-framework