Fix ClassCastException if expected type is Object

[lbilger]

The currently generated code will throw a ClassCastException if type is Object. I'm not quite sure why isAssignableFrom is used here but I tried to keep the change minimal - in my understanding, the result would be the same if the line was

if(Types.getRawType(type).equals(ApiResponse.class)) {

because the ApiResponse only extends Object and implements no interfaces. Maybe it was meant to be ApiResponse.class.isAssignableFrom(Types.getRawType(type))? This would also permit subclasses of ApiResponse, but then it would be more complicated to determine the actual type parameter (because type itself may not be parameterized) and to create the object to return (because it would have to be an instance of the subclass).

Fixes #11509

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.

The build did not work locally, probably because I don't have Java 8 installed anyomore. As it's a really small change, I hope that will be ok...

• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

[wing328]

Tested locally with updated samples and the result is ok

[INFO] Building jar: /Users/williamcheng/Code/openapi-generator2/samples/client/petstore/java/feign/target/petstore-feign-1.0.0-javadoc.jar
[INFO] 
[INFO] --- maven-source-plugin:2.2.1:jar-no-fork (attach-sources) @ petstore-feign ---
[INFO] Building jar: /Users/williamcheng/Code/openapi-generator2/samples/client/petstore/java/feign/target/petstore-feign-1.0.0-sources.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  32.095 s
[INFO] Finished at: 2022-02-05T14:42:21+08:00
[INFO] ------------------------------------------------------------------------

[wing328]

@lbilger thanks for the PR, which has been merged. Next time please follow step 3 in the PR checklist so that the CI can verify the results with updated samples.