swagger -> raml, null primitive type -> nil

[marbemac]

According to the raml10 spec, nil is the allowed type. The JSON schema spec uses the null primitive type, so this is what swagger specs will use. Thus, we need to change null -> nil when parsing types/schemas for raml 10.

RAML10
https://github.com/raml-org/raml-spec/blob/master/versions/raml-10/raml-10.md#built-in-types

I could not find the allowed primitive types for raml08, so I am not sure if null is allowed (or nil). Perhaps @sichvoge could shed some light on this? This is what I could find:
https://github.com/raml-org/raml-spec/blob/master/versions/raml-08/raml-08.md#type

To replicate, here is an example input swagger that will lead to a (invalid) null property in the raml10 export:

{
    "swagger": "2.0",
    "info": {
        "title": "Test",
    },
    "paths": {
        "/users": {
            "get": {
                "responses": {
                    "200": {
                        "description": "",
                        "schema": {
                            "type": "object",
                            "properties": {
                                "foo": {
                                    "type": "null"
                                }
                            }
                        }
                    }
                }
            }
        }
    },
}

[marbemac]

@glodieu could you take a look at this one when you have some time? Let me know if you have any questions!

[sichvoge]

If it is a pure JSON schema, RAML 0.8 does not care if you put in "null". So the following schema should be fine:

"type": "object",
"properties": {
  "foo": {
    "type": "null"
  }
}