Use user-provided timestamp string as-is when calculating server-side signature

[achimnol]

Different languages may use slightly different timezone offset formats when generating ISO 8601-style datetime strings. So, after checking timing range (+- 15 minutes to the server time), we need to use the date header value as-is when canonicalizing the request.

Python 3.6:

>>> from datetime import datetime
>>> from dateutil.tz import tzutc
>>> datetime.now(tzutc()).isoformat()
'2017-01-10T06:15:09.189505+00:00'
>>> datetime.now().isoformat()
'2017-01-10T15:15:15.573437'
>>> datetime.utcnow().isoformat()
'2017-01-10T06:15:23.914917'

Javascript

var d = new Date;
console.log(d.toISOString());
// 2017-01-10T06:16:21.828Z