Description:
------------
mb_convert_encoding fails to convert HTML-ENTITIES encoded chars to the following encodings:

* BASE64
* Quoted-Printable
* 7bit


Test script:
---------------
<?php

foreach ([
    'BASE64',
    'UUENCODE',
    'HTML-ENTITIES',
    'Quoted-Printable',
    '7bit',
    '8bit',
] as $encoding) {
    echo "$encoding: ", mb_convert_encoding('&#x61;', $encoding, 'HTML-ENTITIES'), PHP_EOL;
}


Expected result:
----------------
BASE64: YQ==
UUENCODE: a
HTML-ENTITIES: a
Quoted-Printable: a
7bit: a
8bit: a


Actual result:
--------------
BASE64: JiN4NjE7
UUENCODE: a
HTML-ENTITIES: a
Quoted-Printable: &#x61;
7bit: &#x61;
8bit: a