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

Create an APIView with HEAD allowed:

class HeadAPIView(APIView):
    permission_classes = (AllowAny,)
    allowed_methods = ('POST', 'OPTIONS', 'HEAD')

And then use django-rest-swagger to browse API docs.

Expected behavior

Just work.

Actual behavior

["KeyError: 'head'", '  File "django/core/handlers/base.py", line 149, in get_response', '  File "django/core/handlers/base.py", line 147, in get_response', '  File "django/views/decorators/csrf.py", line 58, in wrapped_view', '  File "django/views/generic/base.py", line 68, in view', '  File "rest_framework/views.py", line 466, in dispatch', '  File "rest_framework/views.py", line 463, in dispatch', '  File "rest_framework/decorators.py", line 52, in handler', '  File "project/urls.py", line 32, in schema_view', '  File "rest_framework/schemas.py", line 78, in __init__', '  File "rest_framework/schemas.py", line 132, in get_api_endpoints', '  File "rest_framework/schemas.py", line 124, in get_api_endpoints', '  File "rest_framework/schemas.py", line 185, in get_key']

This is because schemas.py does not handle unexpected key (method) that is not defined in mapping dict (only GET, POST, PUT, PATCH and DELETE are there):

default_mapping = {
    'get': 'read',
    'post': 'create',
    'put': 'update',
    'patch': 'partial_update',
    'delete': 'destroy',
}

actions = getattr(callback, 'actions', self.default_mapping)
action = actions[method.lower()]

Generally speaking, schemas.py cannot deal with unexpected method such as HEAD, and invalid method such as INVALID.