Added option to set preferred request body content-type

Author: thim81

lib/options.js

@@ -396,7 +396,7 @@ module.exports = {
         description: 'When there are multiple content-types defined in the request body of OpenAPI, the conversion ' +
           'selects the preferred option content-type as request body.If "first-listed" is set, the first ' +
           'content-type defined in the OpenAPI spec will be selected.',
-        external: false,
+        external: true,
         usage: ['CONVERSION'],
         supportedIn: [VERSION20, VERSION30, VERSION31],
         supportedModuleVersion: [MODULE_VERSION.V2, MODULE_VERSION.V1]