Add 409 return status code to Config API resources (#465)

[baniol]

What did you implement:

Modified response status codes from 400 to 409 for POST requests for event types, functions, subscriptions and CORS configs resources. Updated unit tests and API docs accordingly.

How did you implement it:

How can we verify it:

Todos:

I'm not sure if a test for createSubscription in httpapi.go should be added in scope of the task

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: NO
Is it a breaking change?: NO

[mthenw]

@baniol any idea why TravisCI is not running?

[baniol]

I have no idea why Travis is not running for this branch.

[mthenw]

@baniol can you login to travisci.org and check if there is no need to accept some ToS or something?

[baniol]

I don't see anything like that in my account, unfortunately.
I checked https://travis-ci.org/serverless/event-gateway/requests, and I can see my PR failing with "Build type disabled via repository settings (pull_request)". Could this be related?

[codecov]

Codecov Report

Merging #466 into master will increase coverage by 1.47%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
+ Coverage    69.3%   70.78%   +1.47%     
==========================================
  Files          35       35              
  Lines        2163     2163              
==========================================
+ Hits         1499     1531      +32     
+ Misses        602      570      -32     
  Partials       62       62

Impacted Files	Coverage Δ
httpapi/httpapi.go	90.47% <100%> (+7.61%)	⬆️

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[mthenw]

@baniol oh, I see. I fixed that.

[mthenw]

@baniol looks great.