fix(index): export the real toString implementation

[simonkundrik]

Fixes #1870.

toString is listed in the exported object in src/index.js, but the file never imports it. So the shorthand property just picks up the inherited Object.prototype.toString, and the real implementation in src/lib/util/toString.js is never exposed:

const validator = require('validator');

validator.toString('test');                        // '[object Object]'
validator.toString === Object.prototype.toString;  // true

The other to* sanitizers (toDate, toFloat, toInt, toBoolean) are each imported at the top of index.js, exported, and documented in the README. toString was the only one missing its import, and it sits in the export list right between normalizeEmail and isSlug, which is exactly where the import would have gone. So this looks like an import that got dropped rather than an intentional omission.

Adding the import makes it behave like the util it points at:

validator.toString('test');  // 'test'
validator.toString(123);     // '123'
validator.toString(null);    // ''

I went with wiring up the import rather than deleting the export, since that's what the one commenter on the issue asked for, and removing it wouldn't really change anything for callers (validator.toString would still resolve through the prototype). Happy to flip it to a removal if you'd rather shrink the API surface.

Worth noting the existing should export sanitizers test only does a typeof check, which can't catch this since typeof validator.toString was already 'function'. The new test in exports.test.js asserts it isn't the prototype method, so it fails on master and passes here.

Full suite is 292 passing, eslint clean.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d4e02ee) to head (5b49cbc).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2839   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2598      2599    +1     
  Branches       658       658           
=========================================
+ Hits          2598      2599    +1     

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[tux-tn]

LGTM