.stringify() and .fields do not support nthDayOfWeek expressions (e.g. 0 9 * * 1#2)

[Jackman3005]

Note: The library does correctly parse the expression and provide correct next() dates for nthDayOfWeek expressions.

Issue

Given the expression 0 9 * * 1#2 which represent "Once a month on the first Monday of the Month at 9am". When the expression's fields are extracted via .fields and then the expression is rebuilt and stringified, it loses the nthDayOfWeek configuration and results in 0 9 * * 1, which is incorrect.

See the following failing test I wrote and relevant console output:

test("parsing `nthDayOfWeek` expressions to fields and rebuilding from fields does not change the expression", () => {
  // Every month on the second Tuesday at 9am.
  const expression = "0 9 * * 2#2";
  const fields = parseExpression(expression).fields;
  console.log("Fields: ", fields);
  // Fields: {
  //   second: [ 0 ],
  //   minute: [ 0 ],
  //   hour: [ 9 ],
  //   dayOfMonth: [
  //      1,  2,  3,  4,  5,  6,  7,  8,  9,
  //     10, 11, 12, 13, 14, 15, 16, 17, 18,
  //     19, 20, 21, 22, 23, 24, 25, 26, 27,
  //     28, 29, 30, 31
  //   ],
  //   month: [
  //      1,  2, 3, 4,  5,
  //      6,  7, 8, 9, 10,
  //     11, 12
  //   ],
  //   dayOfWeek: [ 2 ]
  // }

  const expressionFromFields = fieldsToExpression(fields).stringify(false);

  expect(expressionFromFields).toEqual(expression); // <--- Expectation fails
  // Expected: "0 9 * * 2#2"
  // Received: "0 9 * * 2"
});

Possible solution

The nthDayOfWeek configuration is stored in the private _options field of the expression. I'm not sure of other consequences that might arise, but it seems to me this should be moved from _options to fields so it can be passed through and used in fieldsToExpression to correctly rebuild the expression state.

[Jackman3005]

Upon further testing I realized that even if you do not convert to fields, the original expression is unable to stringify(..) nthDayOfWeek expressions correctly.

See the following failing test I've written:

test("stringify expressions with `nthDayOfWeek` configuration", async () => {
  // Every month on the second Monday at 9am.
  const expression = "0 9 * * 1#2";

  const parsedExpression = parseExpression(expression);

  expect(parsedExpression.stringify(false)).toEqual(expression);
  // Expected: "0 9 * * 1#2"
  // Received: "0 9 * * 1"
});