proper fix for #2300
Prevent duplicate merge events
Update README
API: extracted helper method to validate required parameters, code clean up

Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.
Merge branch 'master' into fixes/api
API: fixed adding a note

Now the correct attribute is checked if it's available or not. Also fixed a test.