Untranslatable fields are overly sensitive to Blueprint field's key capitalization

Describe the bug
Programmatically updating a model with untranslatable fields can lead to unwanted results if your code specifies a different key case from those in a Blueprint.

To Reproduce
Steps to reproduce the behavior:

Start with a Blueprint with a field like:
```
SOMEID:
type: text
translate: false
```
Programmatically create a page with content resembling:
```
[
'someid' => 'myidentifer'
]
```
Programmatically update the content for a non-default language
```
$page->update(`['foo' => 'bar'], 'fr-fr);
```

The resulting translation's content will resemble:

[
'someid' => 'myidentifier',
'foo' => 'bar',
'SOMEID' => null,
]

Expected behavior I would expect the resulting content to not include 'myidentifier'.

[
  'someid' => null,
  'foo' => 'bar',
]

Kirby Version
3.3.5

getkirby / kirby

Untranslatable fields are overly sensitive to Blueprint field's key capitalization #2577