fix(query): handle $round with just 1 arg re: code review comments

Author: vkarpov15

lib/helpers/query/cast$expr.js

@@ -119,13 +119,10 @@ function _castExpression(val, schema, strictQuery) {
   }
   if (val.$round) {
     const $round = val.$round;
-    if (!Array.isArray($round) || $round.length !== 2) {
+    if (!Array.isArray($round) || $round.length < 1 || $round.length > 2) {
       throw new CastError('Array', $round, '$round');
     }
-    val.$round = [
-      castNumberOperator($round[0], schema, strictQuery),
-      castNumberOperator($round[1], schema, strictQuery)
-    ];
+    val.$round = $round.map(v => castNumberOperator(v, schema, strictQuery));
   }
 
   _omitUndefined(val);

test/helpers/query.cast$expr.test.js

@@ -112,7 +112,10 @@ describe('castexpr', function() {
   it('casts $round (gh-13881)', function() {
     const testSchema = new Schema({ value: Number });
 
-    const res = cast$expr({ $eq: [{ $round: ['$value', '00'] }, 2] }, testSchema);
+    let res = cast$expr({ $eq: [{ $round: ['$value', '00'] }, 2] }, testSchema);
     assert.deepStrictEqual(res, { $eq: [{ $round: ['$value', 0] }, 2] });
+
+    res = cast$expr({ $eq: [{ $round: ['$value'] }, 2] }, testSchema);
+    assert.deepStrictEqual(res, { $eq: [{ $round: ['$value'] }, 2] });
   });
 });