Treat any document/pdf attachment as if it were application/pdf

This is a fix for issue #1232.  Richard Taylor pointed
out that some PDF attachments had the non-standard content-type
document/pdf, and that these weren't being treated as PDFs.
(Ganesh Sittampalam discovered that all of these PDFs were
generated by a Lexmark X945e, according to the PDF metadata.)

This commit adds an extra case to normalise_content_type to map
document/pdf to application/pdf.

In fact, since the upgrade of the Mail gem in ccebe3c3d6d4dc5f81
the behaviour when handling was much better, but this commit
also means that you get the right icon for the attachment, and
can be cherry-picked onto older versions to fix #1232.