Register default converter for Object fields

[remkop]

If a field is defined without a type, and the compiler cannot derive the type from the context, the field's type is Object.

// example scripting class
class Params {
    @Option(names = "-o") option
}

As of picocli v2.0.1, this results in an exception picocli.CommandLine$MissingTypeConverterException: No TypeConverter registered for java.lang.Object of field private java.lang.Object picocli.groovy.CommandLineTest$Params.option.

The solution is to register a type converter for Object.class.