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 the non-standard content-type
document/pdf 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.