Remove reference to QMailCodec to prevent possible confusion
Export function for decoding byte streams using a particular encoding
When decoding from unknown encoding and assuming UTF-8, be sure to not stop at first NULL byte

This does *not* fix #321160 (this code is not even triggered in this situation),
but it's a sane thing to do anyway. I guess that the code shall be audited for
other instances where we might silently truncate the data here; it's caused by
an implicit conversion from QByteArray to uchar*.

CCBUG: 321160
WebKit: convert plaintext input with NULL bytes without silent truncation

This does *not* fix HTML input containing null bytes. I don't know if this
problem is relevant for HTML e-mails as well.

BUG: 321160
REVIEW: 111030