html_entity_decode fix for message passed to swiftmailer

Thanks jgarman!

"This fixes a bug exposed by using PHP versions < 5.4.0. The default
encoding used by PHP versions < 5.4.0 is ISO-8859-1 (Latin-1) whereas
the default encoding used by PHP versions >= 5.4.0 is UTF-8.

The Swift Mailer component expects that the input is well-formed UTF-8,
so passing in Latin-1 data creates malformed UTF-8 input and causes the
Swift Mailer's Quoted Printable MIME encoder to create malformed
output. The bug will manifest itself by copying large quantities of the
original (non-QP encoded) message into the output, causing duplicate
text and other bad artifacts.

An example $message that exposes the bug:

$message =
"A&nbsp;AA&nbsp;AAAAAAAAAAAAAAAAAAAAAAAAAA&nbsp;AAAAAAAAAAAAAAAAAAAAAAAA
AAA";
Passing the $message above into html_entity_decode on PHP versions <
5.4.0 will produce a single byte 0xa0 for the non-breaking space
characters, which is not valid UTF-8."