[TypeScript-Fetch] Invalid interface definition when using additionalProperties

Description

Compiling the following spec into a TypeScript-Fetch client results in invalid code.
Specifically, the generated code contains

export interface GenericMap extends null<String, number> {
}

which the TypeScript compiler rejects.

Swagger-codegen version

I tested it on 2.2.1 (latest stable) and 2.3.0 (7aebcfa).

Swagger declaration file content or url

...
definitions:
  GenericMap:
    additionalProperties:
      type: integer
  Container:
    type: object
    properties:
      works:
        type: object
        additionalProperties:
          type: string
      breaks:
        $ref: "#/definitions/GenericMap"

Command line used for generation

swagger-codegen generate -i test.yaml -l typescript-fetch

Steps to reproduce

Compile a swagger API containing the definitions mentioned above into a typescript-fetch client.

Suggest a Fix

The Container type shows that additionalProperties are correctly handled in some cases (when used for the anonymous type of the works member), but not when they are used for a type which is basically a map. I believe it would be sufficient to emit the following type, without inheriting from any other type:

export interface GenericMap {
  [key: string]: number;
}

Reproduce the issue with the provided YAML and the swagger-codegen generate -i test.yaml -l typescript-fetch command, then trace the typescript-fetch generator's handling of named schemas with additionalProperties. The work is done when the generated GenericMap is valid TypeScript and the generated client compiles without the null<String, number> interface error.