add custom Content-Types to nima

Author: SentryMan

http-generator-nima/src/main/java/io/avaje/http/generator/helidon/nima/ControllerMethodWriter.java

@@ -166,7 +166,9 @@ private void writeContextReturn() {
       case APPLICATION_JSON -> writer.append(contentTypeString + "APPLICATION_JSON);").eol();
       case TEXT_HTML -> writer.append(contentTypeString + "TEXT_HTML);").eol();
       case TEXT_PLAIN -> writer.append(contentTypeString + "TEXT_PLAIN);").eol();
-      case UNKNOWN -> writer.append(contentTypeString + "create(\"%s\"));", produces).eol();
+      case UNKNOWN -> writer
+          .append(contentTypeString + "create(\"%s\"));", producesOp.orElse("UNKNOWN"))
+          .eol();
     }
   }