mbstring: Fix mb_rtrim() for UTF-8 text

[nsfisis]

Component

Polyfill-mbstring

Problem

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

How to Reproduce

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.