General api improvements

[adedw]

• Response body should be application/json, not text/plain;
• Preferably return object due api consistency;
• REST stands for Representational State Transfer, therefore return new state after create/update;
• 201 status code for successfully created objects;
• Include location header for created objects.
• Payload should be in body for create/update.

example:

> POST <base-url>/api/v1/categories HTTP/1.1
> Content-Type: application/json
> Accept: application/json
> Authorization: Bearer <token>
>
> {
>     "categoryName": "Headsets"
> }

< HTTP/1.1 201 Created
< Content-Type: application/json
< Location: <base-url>/api/v1/categories/8d9830cc-2e15-42d7-a1c2-7b0c8b20d1b0
< <rest-headers>
<
< {
<     "uuid": "8d9830cc-2e15-42d7-a1c2-7b0c8b20d1b0",
<     "categoryName": "Headsets"
< }