Restore Prefer header functionality

[yakovkhalinsky]

See PR #61

We've dropped this functionality at some point.

Need to restore the ability to force responses by HTTP code by using the Prefer:status=XXX in the header.

[yakovkhalinsky]

ping @marcelogo 👯

[marcelogo]

@yakovkhalinsky

Drakov supports this functionality. It is just the matter of having an example that uses the Prefer header.

### Retrieve from GET [GET]
First GET example with header 

+ Request

    + Headers

            Custom-header: First


+ Response 200 (application/json;charset=UTF-8)

    + Body

            {"first": "response"}

+ Request

    + Headers

            Custom-header: Second


+ Response 200 (application/json;charset=UTF-8)

    + Body

            {"second": "response"}

+ Request

    + Headers

            Prefer: status=400

+ Response 400 (application/json;charset=UTF-8)
Get examples with a specific status code (eg. 400)

    + Body

            {"error": "Bad request"}

In this example, whenever Drakov receives a GET request with Prefer header and value status=400 a response {"error": "Bad request"} will be sent back.

[senyor-developer]

The documentation says that it's still supported (Changing the API Blueprint adding a custom header does not count ;) )

The feature is very useful for client testing purpose only and does not force you to change the Blueprint, creating confusion confusion to the API users. I have also an example that I can't make it work also with this proposal:

## item [/item/{id}]

+ Parameters
    + id: 5000 (number, required) - Item ID

### DELETE an item [DELETE]

+ Request

    + Headers

            Accept: application/vnd.api+json;version=1

+ Response 204

+ Request

    + Headers
            Accept: application/vnd.api+json;version=1
            Prefer: status=404

+ Response 404 (application/json)

    + Body

            {
              "error": "Not Found",
              "type": 404,
              "message": "item not found" does not exists": ""
            }