Fix request #6316: moving forced utf-8 compliance on regex.

@see http://www.php.net/manual/en/reference.pcre.pattern.modifiers.php
When the subject string contains invalid UTF-8 sequences / codepoints, it basically result in a "quiet death" for the preg_* functions, where nothing is matched but without indication that the string is invalid UTF-8

Change-Id: Ifb1eeafef42a1b9e44ee653341539386d48c83a4
Merge commit 'refs/changes/87/1987/1' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix request #6316: moving forced utf-8 compliance on regex.

Change-Id: Id454fd5216e1f328a4781f022bea723949cc617b
This is Tuleap 6.10.99.57

Change-Id: Ia5660cd0a28348b06ef844a183d3a54ba21a3b2e