Fully flatten an array

[u01jmg3]

This is:

- [x] a bugfix
- [ ] a new feature
- [ ] refactoring
- [ ] additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.

Fixes #2955

[oleibman]

By changing ConcatenateTest and its data, you give the impression that this test directly affects Concatenate, which it does only very indirectly. Please restore ConcatenateTest and its data, and create a separate test for this condition. You might add the test to ConcatenateTest, but I think it would be better added to your new ArrayTest member. Something like:

$spreadsheet = new Spreadsheet();
$sheet1 = $spreadsheet->getActiveSheet();
$sheet1->setTitle('Formula');
$sheet1->fromArray(
    [
        ['Number', 'Formula'],
        ['52101293', '=CONCAT(INDEX(Lookup!$B$2, MATCH($A2, Lookup!$A$2, 0)))'],
    ]
);
$sheet2 = $spreadsheet->createSheet();
$sheet2->setTitle('Lookup');
$sheet2->fromArray(
    [
        ['Number', 'Formula'],
        ['52101293', 'PHP'],
    ]
);
self::assertSame(whatevertheresultshouldbe, $sheet1->getCell('B2')->getCalculatedValue());
$spreadsheet->disconnectWorksheets();

Note that it is not necessary for you to use try/catch. If the calculation results in an exception (or issues a notice), the test will not succeed, which is sufficient for our purposes.

[u01jmg3]

Done 👍🏻

[oleibman]

Thank you for your contribution.