Add materials for JSON.MERGE command.

[simonprickett]

When the JSON.MERGE command is available in the cloud, enhance the course with content for it.

[simonprickett]

Do this at the same time as #60

[simonprickett]

This should go in the course in or just after section 1.4 - Updating Data with RedisJSON.

[simonprickett]

We should also update the code examples in section 2.2 to demonstrate this.

[simonprickett]

Command examples - remove author, change content of editions, update pages, add has_ebook_version:

127.0.0.1:6379> JSON.MERGE ru204:book:274 $ '{"pages": 350, "editions": [ "english", "spanish", "french"], "author": null, "has_ebook_version": true}'
OK

Add the author back, remove has_ebook_version:

127.0.0.1:6379> JSON.MERGE ru204:book:274 $ '{"author": "S.E. Grove", "has_ebook_version": null }'
OK

[simonprickett]

Also add example for when the key doesn't exist and when the path into the document doesn't exist but the document does.

[simonprickett]

Add a fragment of a new book (where the key didn't exist already):

Check if the book exists...

127.0.0.1:6379> EXISTS ru204:book:9999
(integer) 0

Demo that you can only merge into the root when the key doesn't exist yet:

127.0.0.1:6379> JSON.MERGE ru204:book:9999 $.metrics '{"rating_votes": 22}'
(error) ERR new objects must be created at the root

Create a partial new book there:

127.0.0.1:6379> JSON.MERGE ru204:book:9999 $ '{"author": "Redis", "id": "9999"}'

OK

Tidy up:

127.0.0.1:6379> DEL ru204:book:9999
(integer) 1

[simonprickett]

Also need to note that if you want to set a value to explicitly null you should use JSON.SET or JSON.MSET as JSON.MERGE will delete the value and its key completely.