Fix that Message#attachments cannot count singlepart attachment

[kirikak2]

see #1143

[HashNotAdam]

Unfortunately, this issue is causing me problems when parsing DMARC reports coming from Google. I've tested this branch and, apart from correcting the above-mentioned typo, found that it worked without modification.

@kirikak2 any chance you can get this over the line? Alternatively, I could make a new PR based on this one but compatible with the current version of mail