Fix/mail connector read message contents

[TobiasAdler]

Description

When an Email object was created, it was possible that a NullPointerException could be thrown, if the disposition of a part of this multipart message was unknown. The if-Statement for the distinction of the part (either INLINE or ATTACHMENT) was replaced to work reliably, even if the disposition is unknown. In addition some typos in MailConnectorTest were fixed.

Fixes #371

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[martingrossmann]

@TobiasAdler Looks good for me. Please validate that solution with mails generated from different mail clients.

[TobiasAdler]

@martingrossmann The solution was tested successfully, with and without attachments, with multiple different mail clients, including Microsoft Outlook (native & web app), Gmail (mobile & web app) and Thunderbird. All of these mails could be queried without any problems and all contents of these mails, the message itself as well as all attachments, were intact and could be saved and verified with the available methods.