Replace manually configured RFC3339 DateFormatter instances with ISO8601DateFormatter

Expected behavior

It should be possible to replace manually configured RFC3339 DateFormatter instances with ISO8601DateFormatter.

Actual behavior

Examples include both NSDate+WordPressJSON.m & WPStatsServiceRemote.m

Steps to reproduce the behavior

N/A

WordPressKit Environment

• WordPressKit Version : WordPressKit v2.1.1 & before
• OS : iOS 12.x
• Device : all devices
• Xcode Version : Xcode 10.2 beta

Additional Context

N/A