Merge PATCH doesn't work on permissions

[glasserc]

$ http -a 'user:pass' http://localhost:8888/v1/buckets/b4/collections/cc
HTTP/1.1 200 OK
Access-Control-Expose-Headers: Expires, Content-Length, Pragma, Backoff, Cache-Control, Alert, Retry-After, ETag, Last-Modified
Cache-Control: no-cache, no-store
Content-Length: 182
Content-Type: application/json
Date: Fri, 18 Aug 2017 16:52:04 GMT
Etag: "1503075082664"
Last-Modified: Fri, 18 Aug 2017 16:51:22 GMT
Server: waitress
X-Content-Type-Options: nosniff

{
    "data": {
        "a": "whoo",
        "id": "cc",
        "last_modified": 1503075082664
    },
    "permissions": {
        "read": [
            "hi"
        ],
        "write": [
            "basicauth:08eee2542a2328390dfaa7390fa7df0d64a7081b44d629bfacd9b4e02bd2676d"
        ]
    }

Let's get rid of the read permissions.

$ http -a 'user:pass'  PATCH http://localhost:8888/v1/buckets/b4/collections/cc "Content-Type:application/merge-patch+json" data:='{"a": null}' permissions:='{"read": null}'
HTTP/1.1 400 Bad Request
Access-Control-Expose-Headers: Alert, Backoff, Content-Length, Retry-After
Content-Length: 215
Content-Type: application/json
Date: Fri, 18 Aug 2017 16:52:47 GMT
Server: waitress
X-Content-Type-Options: nosniff

{
    "code": 400,
    "details": [
        {
            "description": "\"None\" is not iterable",
            "location": "body",
            "name": "permissions.read"
        }
    ],
    "errno": 107,
    "error": "Invalid parameters",
    "message": "permissions.read in body: \"None\" is not iterable"
}

Note that JSON Patch does work by applying changes uniformly to data and permissions.

[gabisurita]

Null read permissions is failing colander validation. This is easy to fix if we allow {"permissions": {"read": null} on PermissionsSchema, but that poses another problem: should we allow permissions to be null on "regular" JSON? Which would be the behavior? Do nothing? Or should we add some custom validation to raise an exception?

[gabisurita]

related to Pylons/colander#299

[glasserc]

Hmm, I think it's OK to disallow it for "regular" JSON. I like the solution in your PR.