[Intl] Fixed a few bugs in TextBundleWriter

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

See the included test cases for more information. This code was extracted from #9206.
this does not check that they are ascending actually.

The way we check this in other places in Symfony is using ``keys === range(0, count($keys) - 1)``