[JAVA] Generated api client doesn't work for plain text body types

Created by: Kiolali

see https://github.com/swagger-api/swagger-codegen/issues/8288

tested with openapi-generator 4.0.0

java -jar .\modules\openapi-generator-cli\target\openapi-generator-cli. jar generate -i http://localhost:8080/.../openapi.json -g java -o outputdir --artifact-id openapi-todol ist-client