Description:
------------
mb_detect_encoding doesn't detect string with Windows-1252 ( cp1252 ) encoding. 

The function is supposed to return the first compatible encoding within the provided encoding list as second param.

Even with an ASCII string like 'aaa', it refuse the Windows-1252 encoding.

Test script:
---------------
<?php
$enc = 'Windows-1252';
if (!in_array($enc, mb_list_encodings())) {
    die($enc.' not supported');
}

$val = '100 '. 0x80; // € sign in cp1252
echo mb_detect_encoding($val, $enc.',ISO-8859-1', true).PHP_EOL;

$val = 'aaa';
echo mb_detect_encoding($val, $enc.',ISO-8859-1', true).PHP_EOL;

Expected result:
----------------
Windows-1252
Windows-1252

Actual result:
--------------
ISO-8859-1
ISO-8859-1