Creating a comment to an existing merge request via API without providing a note
returns a status code 400 now, suggesting a bad request. The reason for this
is the resource itself (MR) exists but the required property is not set.
Using the API library to create or update a merge request at the moment a 404 error is returned.
This is fine when the merge request in question does not exist, but does not provide good
information that for example a required attribute is missing.
A status code of 400 (Bad request) is returned when creating or updating a merge request
when either `source_branch` or `target_branch` is missing. A status code of 409 is returned
when `source_branch` and `target_branch` are the same. Tests are added for these cases.
