code: public <T> Configuration registerConverter(ContentConverter<? super T> converter,
                                                 Class<T> classToConvert)
      {
          converters.add(new Converter<>(classToConvert, converter));
          return this;
      }
docs: Registers a converter.
  <p>
  Converters are used mainly by the
  {@link net.sargue.mailgun.content.Builder#text(Object)} method.
  <p>
  They are matched in declaration order checking for assignability
  (inheritance). So you can have a a {@link java.util.Date} converter
  which will be applied to a {@link java.sql.Timestamp} object. You will
  want to register the more specific converters first.
  
  @param <T>            the type parameter
  @param converter      the converter
  @param classToConvert the class to convert
  @return the configuration