[Java] Client - Missing import in JSON.java when using `Object` as schema name

Description

When defining an Object as schema the io.swagger.client.JSON file doesn't compile anymore due to an ambigous usage of Object

Compile error message:

Error:(1397, 29) java: reference to Object is ambiguous
  both class io.swagger.client.model.Object in io.swagger.client.model and class java.lang.Object in java.lang match

// Line 1397
public String serialize(Object obj) {
        return gson.toJson(obj);
    }

Swagger-codegen version

3.0.19

Swagger declaration file content or url

https://pastebin.com/x85KPJtC

Command line used for generation

java -jar swagger-codegen-cli-3.0.19.jar generate -i openapi.json -l java -o clients/java

Steps to reproduce

1. Create a client of a schema which contains Object as schema name
2. Try to compile the client

Suggest a fix/enhancement

The import to java.lang.Object should be added to JSON.java in this case

Generate the Java client with swagger-codegen-cli 3.0.19 using the supplied OpenAPI declaration, then inspect clients/java JSON.java around line 1397. Reproduce the compilation error with a schema named Object; done means the generated client compiles without the ambiguous Object reference.