Lack of X-Slack-Request-Timestamp and/or X-Slack-Signature in requests results in 500

[SpamapS]

Description

I noticed that a test I wrote to simulate a user probing the events URL returned a 500 when I would have expected a 400.

What type of issue is this? (place an x in one of the [ ])

• [x] bug
• [ ] enhancement (feature request)
• [ ] question
• [ ] documentation related
• [ ] testing related
• [ ] discussion

Requirements

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them. -- However, Spotify, my employer, has not yet signed the CLA to my knowledge, so I will not be able to submit a patch until they do.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

Bug Report

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Reproducible in:

slackeventsapi version: 2.2.0 python version: 3.6 OS version(s): irrelevant, but Linux and MacOS

Steps to reproduce:

1. Make a POST request to a slack events URL without X-Slack-Events-Timestamp in the headers.
2. Observe response of 500 and subsequent TypeError raised

Expected result:

A 400 error.

Actual result:

500 and unhandled exception.

Attachments:

[SpamapS]

Apologies for submitting without the form filled out, I have completed it now. The fix is quite simple, but I will need to get Spotify to sign the CLA (or confirm that they already have) to fix it myself.

[SpamapS]

This also affects lack of Signature, though that may want to be a separate issue as it is an entirely different part of the code that assumes this will be set. :)

[SpamapS]

Going deeper, it seems like the only thing this library does on errors is raise exceptions, despite the few places it appears to want to send a 403. Perhaps I missed a doc?

[seratch]

@SpamapS Thanks for the feedback! I just made a pull request to address this. A new patch version including the fix will be out once I've got code reviews from other maintainers.