Exception thrown in List<T> or IDictionary<TKey, TValue> when using interpolation

[NightOwl888]

A simple interpolation attempt:

var x = new J2N.Collections.Generic.List<string>()
            {
                "nothing",
                "else",
                "matters"
            };
            string y = $"{x}";

Yields the following exception:

  Message: 
    System.ArgumentNullException : Value cannot be null.
    Parameter name: format
  Stack Trace: 
    String.FormatHelper(IFormatProvider provider, String format, ParamsArray args)
    CollectionUtil.ToString[T](IFormatProvider provider, String format, ICollection`1 collection)
    StringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ParamsArray args)
    String.FormatHelper(IFormatProvider provider, String format, ParamsArray args)
    String.Format(String format, Object arg0)