Schemas Generator produces error for views with no pagination class.

[f1shear]

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

Class TaskList(generics.ListCreateAPIView):
    queryset =  TaskModel.objects.all()
    serializer_class = TaskSerializer
    pagination_class = None


class ApiDocView(APIView):

    def get(self, request, format=None):
        generator = schemas.SchemaGenerator(title='API')
        return Response(generator.get_schema(request=request))

Expected behavior

Schema generator should consider views with pagination_class = None.
This issue is similar to #4265

Actual behavior

Schema generator gives error
File "/path/to/env/local/lib/python2.7/site-packages/rest_framework/schemas.py", line 284, in get_pagination_fields
paginator = view.pagination_class()
TypeError: 'NoneType' object is not callable