Invaild email subject when containing question mark character

When sending an email with a subject line containing a question mark character the subject line is not properly encoded. More precisely, question mark characters are not encoded by PHP's quoted_printable_encode function. phpBB will need to encode those characters itself.

The quickest way to reproduce the error is to email another user or use the contact us form to send an email with a subject line containing question marks.

Example subject line: Test?

Incorrect encoding

=?US-ASCII?Q?Test??=

Correct encoding

=?US-ASCII?Q?Test=3F?=