Spec-compliance fix in GetStringOrNumberOption

See: #1411

Author: ptomato

lib/ecmascript.mjs

@@ -680,8 +680,12 @@ export const ES = ObjectAssign({}, ES2020, {
       default: // fall through if option not given
     }
     let digits = options.fractionalSecondDigits;
-    if (digits === undefined || digits === 'auto') return { precision: 'auto', unit: 'nanosecond', increment: 1 };
-    digits = ES.ToNumber(digits);
+    if (digits === undefined) digits = 'auto';
+    if (ES.Type(digits) !== 'Number') {
+      digits = ES.ToString(digits);
+      if (digits === 'auto') return { precision: 'auto', unit: 'nanosecond', increment: 1 };
+      throw new RangeError(`fractionalSecondDigits must be 'auto' or 0 through 9, not ${digits}`);
+    }
     if (NumberIsNaN(digits) || digits < 0 || digits > 9) {
       throw new RangeError(`fractionalSecondDigits must be 'auto' or 0 through 9, not ${digits}`);
     }