Description:
------------
"vie" and 'lun' do not end with a dot, but should, as described in
http://demo.icu-project.org/icu-bin/locexp?d_=en&_=es_MX

Also, es_ES returns uppercased first letter, while es_MX does not. This should be investigated

Test script:
---------------
$formatter = new \IntlDateFormatter(
    'es_MX',
    \IntlDateFormatter::NONE,
    \IntlDateFormatter::NONE,
    ini_get('date.timezone')
);

$formatter->setPattern('ccc');

var_dump([
    $formatter->format(new DateTime('2018-09-05')),
    $formatter->format(new DateTime('2018-09-06')),
    $formatter->format(new DateTime('2018-09-07')),
    $formatter->format(new DateTime('2018-09-08')),
    $formatter->format(new DateTime('2018-09-09')),
    $formatter->format(new DateTime('2018-09-10')),
    $formatter->format(new DateTime('2018-09-11')),
]);


Expected result:
----------------
  0 => string 'Mié.' (length=5)
  1 => string 'Jue.' (length=4)
  2 => string 'Vie.' (length=3)
  3 => string 'Sáb.' (length=5)
  4 => string 'Dom.' (length=4)
  5 => string 'Lun.' (length=3)
  6 => string 'Mar.' (length=4)