fix: date localestring format error

[Pcrab]

Since node v18.13.0, ICU is updated to 72.1, which changes the behavior of 'en-CA' toLocaleString.

• ToLocaleDateString inconsistent/wrong output locale to "en-CA"  nodejs/node#45945
• https://github.com/nodejs/node/releases/tag/v18.13.0

Its output could be formatted to YYYY-MM-DD HH:mm:ss, but now it becomes MM/DD/YYYY HH:mm:ss, which is no longer a valid ISO 8601 representation.

The Date.prototype.toJSON or Date.prototype.toISOString is guaranteed to produce ISO 8601 format string, so we can use these methods instead.

[coveralls]

Coverage: 93.514%. Remained the same when pulling b6c0d76 on Pcrab:fix-test into 511ba4f on hexojs:master.

[stevenjoezhang]

Thanks!