This PR adds endpoint PUT /api/v2/cases/{identifier} to update a case.

Other work done:

• fixed /api/v2/cases/{case_identifier}/tasks/{task_identifier} to return 200 instead of 201 on success (added a test)
• removed strict_slashes=False option to all endpoints that had it (we want to be strict) (added a test)
• changed tuple into a single value for business method cases_create (tuple as return values are a code smell)
• changed last bp into blueprint and some id into identifier (avoid abbreviations)
• added several tests about the case update

Remark: I noticed one inconsistency between the cases endpoint. They return case objects, with field case_customer_id. However endpoints POST /api/v2/cases and PUT /api/v2/cases/{identifier} expect field case_customer. I tried to change everything to case_customer_id. But it was too difficult to do, without risking to break past behavior. Maybe we should create an issue to track this and correct later.