[PHP] Incorrect return types if multiple responses are defined

[fMads]

Description

If you define multiple responses, the codegen will only support the first one (typically 200).

openapi-generator version

3.0.0-SNAPSHOT

OpenAPI declaration file content or url

  /path
    post:
      responses:
        200:
          schema:
            $ref: '#/definitions/EmptyResponse'
        202:
          schema:
            $ref: '#/definitions/InBackgroundResponse'

Suggest a fix/enhancement

Correctly generate the return-object based on the HTTP Status Code in the Response.
Also fix ReturnType info in DocBlock

[ackintosh]

@fMads #128 is merged! Thanks for your contribution.