Permissions are not validated on PATCH

[leplatrem]

This is a side effect of #358.

With PUT:

echo '{"permissions": {"read": "me"}}' | http PUT "http://0.0.0.0:8888/v1/buckets/default/collections/articles/records/ff7db2ad-cc85-4715-b563-1579d4c27c3b" --auth=user:pass
HTTP/1.1 400 Bad Request
Access-Control-Expose-Headers: Backoff, Retry-After, Alert
Content-Length: 244
Content-Type: application/json; charset=UTF-8
Date: Fri, 10 Jul 2015 10:55:29 GMT
Server: waitress

{
    "code": 400, 
    "details": [
        {
            "description": "data is missing", 
            "location": "body", 
            "name": "data"
        }, 
        {
            "description": "\"me\" is not iterable", 
            "location": "body", 
            "name": "permissions.read"
        }
    ], 
    "errno": 107, 
    "error": "Invalid parameters", 
    "message": "data is missing"
}

With PATCH:

echo '{"permissions": {"read": "me"}}' | http PATCH "http://0.0.0.0:8888/v1/buckets/default/collections/articles/records/ff7db2ad-cc85-4715-b563-1579d4c27c3b" --auth=user:pass 
HTTP/1.1 200 OK
Access-Control-Expose-Headers: Backoff, Retry-After, Alert
Content-Length: 219
Content-Type: application/json; charset=UTF-8
Date: Fri, 10 Jul 2015 10:56:01 GMT
Etag: "1436525081662"
Last-Modified: Fri, 10 Jul 2015 10:44:41 GMT
Server: waitress

{
    "data": {
        "id": "ff7db2ad-cc85-4715-b563-1579d4c27c3b", 
        "last_modified": 1436525081662, 
        "title": "toto"
    }, 
    "permissions": {
        "read": [
            "e", 
            "m"
        ], 
        "write": [
            "basicauth:631c2d625ee5726172cf67c6750de10a3e1a04bcd603bc9ad6d6b196fa8257a6"
        ]
    }
}

Or:

echo '{"permissions": 32}' | http PATCH "http://0.0.0.0:8888/v1/buckets/default/collections/articles/records/ff7db2ad-cc85-4715-b563-1579d4c27c3b" --auth=user:pass
HTTP/1.1 500 Internal Server Error
Content-Length: 196
Content-Type: application/json; charset=UTF-8
Date: Fri, 10 Jul 2015 10:56:35 GMT
Server: waitress

{
    "code": 500, 
    "errno": 999, 
    "error": "Internal Server Error", 
    "info": "https://github.com/mozilla-services/cliquet/issues/", 
    "message": "A programmatic error occured, developers have been informed."
}

Once #374 is fixed, the protected resource should validate the PATCH payload against the resource mapping.