bug #507 mbstring: Fix mb_rtrim() for UTF-8 text (nsfisis)

This PR was merged into the 1.x branch.

Discussion
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mbstring: Fix mb_rtrim() for UTF-8 text

# Component

Polyfill-mbstring

# Problem

`mb_rtrim()` returns an empty string for UTF-8 text.

# How to Reproduce

```php
echo \Symfony\Polyfill\Mbstring\Mbstring::mb_rtrim('　あいうえお　');
// => '' (empty string)
```

Expected result: `'　あいうえお'`

# Cause

The regular expression used in `mb_rtrim()` implementation does not have `u` flag.

# Changes

* Add `u` flag to the regular expression used in `mb_rtrim()` implementation as `mb_trim()` and `mb_ltrim()` do.
* Add tests.

Commits
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65d1260 mbstring: Fix mb_rtrim() for UTF-8 text

src/Mbstring/Mbstring.php

@@ -983,7 +983,7 @@ public static function mb_ltrim(string $string, ?string $characters = null, ?str
 
     public static function mb_rtrim(string $string, ?string $characters = null, ?string $encoding = null): string
     {
-        return self::mb_internal_trim('{[%s]+$}D', $string, $characters, $encoding, __FUNCTION__);
+        return self::mb_internal_trim('{[%s]+$}Du', $string, $characters, $encoding, __FUNCTION__);
     }
 
     private static function mb_internal_trim(string $regex, string $string, ?string $characters, ?string $encoding, string $function): string

tests/Mbstring/MbstringTest.php

@@ -886,6 +886,7 @@ public static function mbLTrimProvider(): iterable
         yield [' test ', ' test ', ''];
 
         yield ['いああああ', 'あああああああああああああああああああああああああああああああああいああああ', 'あ'];
+        yield ['あいうえお　', '　あいうえお　'];
 
         yield ['漢字', "\u{FFFE}漢字", "\u{FFFE}\u{FEFF}"];
         yield [' abcd ', ' abcd ', ''];
@@ -902,6 +903,7 @@ public static function mbRTrimProvider(): iterable
         yield ['                                                                                                                                 a', str_repeat(' ', 129).'a'];
 
         yield ['あああああああああああああああああああああああああああああああああい', 'あああああああああああああああああああああああああああああああああいああああ', 'あ'];
+        yield ['　あいうえお', '　あいうえお　'];
 
         yield [' abcd ', ' abcd ', ''];