Inline Option enum requires null added to the list of variant

[Johnabell]

According to the discussion here, when we have an enum which is nullable we should add null as one of the enum values.
json-schema-org/json-schema-spec#258

Here is an example of this change being implemented in another library: vega/ts-json-schema-generator#22

Details

Given the following types:

#[derive(Debug, Deserialize, Serialize, PartialEq, Eq, Clone, Copy, JsonSchema)]
enum FunctionalStatus {
    InService,
    OutOfService,
}

#[derive(Debug, Default, Deserialize, Serialize, PartialEq, Clone, JsonSchema)]
struct Example {
    functional_status: Option<FunctionalStatus>
} 

When we generate the schema without inlining the definitions, we get

{
  "functional_status": {
    "description": "Functional status",
    "anyOf": [
      {
        "$ref": "#/definitions/FunctionalStatus"
      },
      {
        "type": "null"
      }
    ]
  },
}

This is correct and successfully validates request when null is given.

However, if we inline definitions we are given this schema,

{ 
  "functional_status": {
    "description": "Functional status",
    "type": [
      "string",
      "null"
    ],
    "enum": [
      "InService",
      "OutOfService"
    ]
  }
}

which fails to validate when you pass a null value with the following error null is not one of ["InService","OutOfService"].

Instead, the schema that is generated should have the following change

 { 
   "functional_status": {
     "description": "Functional status",
     "type": [
       "string",
       "null"
     ],
     "enum": [
       "InService",
-      "OutOfService"
+      "OutOfService",
+      null
     ]
   }
 }