Replace manually configured RFC3339 DateFormatter instances with ISO8601DateFormatter

### Expected behavior

It should be possible to replace manually configured RFC3339 `DateFormatter` instances with [`ISO8601DateFormatter`](https://developer.apple.com/documentation/foundation/iso8601dateformatter).

### Actual behavior

Examples include both [`NSDate+WordPressJSON.m`](https://github.com/wordpress-mobile/WordPressKit-iOS/blob/260d5420b9e07c90ef2421f372de63b55175604a/WordPressKit/NSDate%2BWordPressJSON.m#L5-L16) & [`WPStatsServiceRemote.m`](https://github.com/wordpress-mobile/WordPressKit-iOS/blob/260d5420b9e07c90ef2421f372de63b55175604a/WordPressKit/WPStatsServiceRemote.m#L58-L61)

### 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


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Replace manually configured RFC3339 DateFormatter instances with ISO8601DateFormatter #112

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Uh oh!

Replace manually configured RFC3339 DateFormatter instances with ISO8601DateFormatter #112

Description

Expected behavior

Actual behavior

Steps to reproduce the behavior

WordPressKit Environment

Additional Context

Metadata

Metadata

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