Make defaults for html_entity_decode explicit.

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.