honour Date object passed as legacy 2nd arg to isAfter / isBefore

[HrachShah]

What this PR fixes

isAfter and isBefore silently ignore a Date object that callers pass as
the legacy second argument. The disambiguation between "this is the legacy date
arg" and "this is the new options object" was a typeof options === 'object'
check, which is true for any Date as well, so the new-options branch always
won. isAfter('2010-01-02', new Date('2010-01-01')) returned false even
though 2010-01-02 is plainly after 2010-01-01, because the code then did
options.comparisonDate (which is undefined on a Date), fell back to
Date().toString() for "now", and compared against the current moment.

Repro

import isAfter from 'validator/lib/isAfter';

const pivot = new Date('2010-01-01T00:00:00Z');
isAfter('2010-01-02', pivot); // false on master, should be true
isAfter('2009-12-31', pivot); // true on master, should be false

Fix

Hoist the disambiguation into a small resolveComparisonDate helper that
recognises a Date instance directly, and short-circuit toDate() when the
comparison date is already a Date. The typeof === 'object' branch is also
guarded against null for the same reason the rest of the codebase already
guards null options.

function resolveComparisonDate(options) {
  if (options instanceof Date) {
    return options;
  }
  if (typeof options === 'object' && options !== null) {
    return options.comparisonDate;
  }
  return options;
}

const comparisonDate = resolveComparisonDate(options) || Date().toString();
const comparison = comparisonDate instanceof Date
  ? comparisonDate
  : toDate(comparisonDate);

Tests

The shared test helper in test/testFunctions.js serialises args via
JSON.stringify for error messages, which would strip the Date identity. The
new cases are written as direct assertions against the validator instead.

• test/validators/isAfter.test.js — new '(legacy syntax with Date object)'
  describe block:
  • isAfter('2010-01-02', new Date('2010-01-01')) is true
  • isAfter('2009-12-31', new Date('2010-01-01')) is false
  • isAfter('2010-01-02', new Date('not a date')) is false
• test/validators/isBefore.test.js — same three cases for isBefore.

Both new test blocks fail on master (Stashed the source changes and ran the suite: 2 failing) and pass on the fix (320 passing).

$ npx mocha --reporter dot --require @babel/register --recursive test
  320 passing (247ms)

ESLint passes on the modified files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e36b279) to head (f331d12).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2783   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2586      2599   +13     
  Branches       656       660    +4     
=========================================
+ Hits          2586      2599   +13     

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