[StringHelpers] Add support for Calendar in dateFormat

[LeBezout]

Hi,

In com.github.jknack.handlebars.helper.StringHelpers for the dateFormat helper we can add the support for (the good old) Calendar as already exists for (the good old) Date :

Actual :

private TemporalAccessor toTemporalAccessor(final Object value) {
  if (value instanceof TemporalAccessor) {
	return (TemporalAccessor) value;
  } else if (value instanceof Date) {
	return ((Date) value).toInstant();
  } else {
	// ...
}

Proposal :

private TemporalAccessor toTemporalAccessor(final Object value) {
  if (value instanceof TemporalAccessor) {
	return (TemporalAccessor) value;
  } else if (value instanceof Date) {
	return ((Date) value).toInstant();
  } else if (value instanceof Calendar) {
	return ((Calendar) value).toInstant();
  } else {
	// ...
}