OpenAPI Schema inconsistent operationId casing

[peterfarrell]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

There is inconsistent casing of titles in the the OpenAPI operationId naming.

Expected behavior

Expected behavior is that the method names are consistent in case between them. I.e.

RetrieveRegistrant
ListRegistrant

or

retrieveRegistrant
listRegistrant

Given that the rest of the method names are lowercase, I would opt for listRegistrant (initial lower camel case) over CamelCase.

Actual behavior

It lowercases all methods names accept for List which is hardcoded.

retrieveRegistrant
ListRegistrants

Problem area:

[https://github.com/encode/django-rest-framework/blob/master/rest_framework/schemas/openapi.py#L112](https://github.com/encode/django-rest-framework/blob/master/rest_framework/schemas/openapi.py#L112

[carltongibson]

Hi @peterfarrell. Want to make a PR correcting that L?

[domingues]

We still have inconsistency when using APIView:

• ViewSet:
  
• APIView:
  

One option is to lower case in this line:

django-rest-framework/rest_framework/schemas/openapi.py

Line 112 in 8a5bcf9

action = self.method_mapping[method.lower()]