Not handling headers for sent/deleted messages in OSTs

[gwiedeman]

Describe the bug
Sent messages and deleted items in Outlook seem to often only have partial headers. The MSG parser handles these well, but the PST parser is not parsing these headers correctly. This is because the .transport_headers attribute provided by libpff just returns None here.

This affects EML and MBOX derivatives. For now, the parser just makes a new email.message.Message object with only one header: headers: not found

To Reproduce
Steps to reproduce the behavior:

1. Use mailbagit on an OST file containing deleted or sent items.
2. Select eml derivatives
3. Get EMLs with only one header: headers: not found

Expected behavior
Should return a headers object with the few headers these messages contain.

Environment (please complete the following information):
_ GUI or command line?: CLU

• OS: [e.g. iOS] Win10
• Python version: 3.7.3