fix(php7.1) Fix iconv UTF-8 conversion Notice

[carlbastos]

See phpearth/docker-php#3 for further details.

You can reproduce/test by running php -a inside the php-fpm container and then executing the following function:

iconv('UTF-8', 'ASCII//TRANSLIT//IGNORE', 'šěýčíéáýřčíšýíščř');

before the fix, the following Notice will pop:

Notice: iconv(): Wrong charset, conversion from `UTF-8' to `ASCII//TRANSLIT//IGNORE' is not allowed in php shell code on line 1

after the fix, no output will be printed.