Upserting to memory database only inserts

[jrmullins]

Describe the bug
Upserting policies, with the same IDs/data, to the memory database, causes duplicate records, instead of updating.

To Reproduce
Steps to reproduce the behavior:

1. Run DATABASE_URL=memory go run main.go serve
2. PUT the same policy more than once

PUT /engines/acp/ory/exact/policies HTTP/1.1
Host: localhost:4466
Content-Type: application/json
{
  "actions": [
    "delete"
  ],
  "description": "Delete first blog post",
  "effect": "allow",
  "id": "alice-deletes-blog-post",
  "resources": [
    "blog_posts:my-first-blog-post"
  ],
  "subjects": [
    "alice"
  ]
}

3. Get the list of policies and withness duplicate entries

GET /engines/acp/ory/exact/policies HTTP/1.1
Host: localhost:4466
Content-Type: application/json

Expected behavior
Upserting a policy will update a pre-existing policy based on the ID. If this is a limitation of the memory database, I would expect some sort of warning.

Version:

• Go version 1.11
• Keto origin/master @ ed7af3f