[RustServer] `object` type is generated when an empty object is given

Description

When the object data type is empty (or have nothing understandable to the generator), it generates object type, which is invalid to rust.

      parameters:
        - name: param
          in: body
          description: request body
          required: true
          schema:
            type: object
            additionalProperties:
              type: string

to

 fn test_inline_additional_properties(&self, param_param: object, context: &Context) -> Box<Future<Item=TestInlineAdditionalPropertiesResponse, Error=ApiError> + Send> {
                                                          ^^^^^^ invalid type object

Swagger-codegen version

master (dc69138de52f089f9a7ad8be475ebbaa8c6e2d99)

Swagger declaration file content or url

swagger: '2.0'
info:
  description: "This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\"
  version: 1.0.0
  title: Swagger Petstore
paths:
  /fake/inline-additionalProperties:
    post:
      tags:
        - fake
      summary: test inline additionalProperties
      description: ''
      operationId: testInlineAdditionalProperties
      consumes:
        - application/json
      parameters:
        - name: param
          in: body
          description: request body
          required: true
          schema:
            type: object
            additionalProperties:
              type: string
      responses:
        '200':
          description: successful operation

Command line used for generation

$ java -jar swagger-codegen-cli.jar generate  -v -l rust-server -i swagger.yaml   -o server

Steps to reproduce

Just run the command above

Related issues/PRs

None

Suggest a fix/enhancement

Just handle object types around here. I think serde_json::Value is suitable. I'll do it.

Start at modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/RustServerCodegen.java around line 128, then reproduce the issue with the provided Swagger declaration and rust-server generation command. Verify that the inline additionalProperties body parameter no longer produces the invalid Rust type object and uses the intended valid representation.