Respond with 411 Length Required when missing

[gregersrygg]

What does this PR do and why is it necessary?

Changes HTTP response code for file upload to 411 (Length Required) instead of 400 (Bad Request) when Content-Length is missing. It is very hard for the client to guess that length is required by the server when the response doesn't say anything about why it's a bad request, and it's perfectly valid to do a file upload without a Content-Length in the header. Using 411 response code tells the client why it's a bad request.

How was it tested? How can it be tested by the reviewer?

I don't know python, and the documentation is not good enough to explain how to setup and run the unit tests. The only thing this change will possibly break is an already broken client, or a unit test that is hard-coded to expect 400 (which is essentially the same as 411 - just less informative).

Any background context you want to provide?

https://tools.ietf.org/html/rfc7231#section-6.5.10

[foosel]

Very good point. I took the liberty to cherry-pick this to the maintenance branch since it's not a new feature but rather an improvement or even a bug fix and should hence go with the next stable release 1.3.6. Github doesn't properly detect cherry picks, so I'm closing this PR manually. Your commit is merged as b002e41.

the documentation is not good enough to explain how to setup and run the unit tests

I've also fixed that on the maintenance branch, already visible on that branch on docs.octoprint.org. The contribution guidelines have also been updated accordingly on maintenance and that will go live with the next release.