Fix date encoding/decoding

[cbaker6]

The date formatting for encoding/decoding dates was not being set properly, causing times to match on the server.

Apple recommends the following for fixed format date representations:

let RFC3339DateFormatter = DateFormatter()
RFC3339DateFormatter.locale = Locale(identifier: "en_US_POSIX")
RFC3339DateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ" //Parse uses "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'" instead
RFC3339DateFormatter.timeZone = TimeZone(secondsFromGMT: 0)

Fix #102

[codecov]

Codecov Report

Merging #103 (5da1268) into main (96b26a5) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head 5da1268 differs from pull request most recent head afc29c2. Consider uploading reports for the commit afc29c2 to get more accurate results

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
- Coverage   81.05%   81.00%   -0.06%     
==========================================
  Files          65       65              
  Lines        5480     5474       -6     
==========================================
- Hits         4442     4434       -8     
- Misses       1038     1040       +2     

Impacted Files	Coverage Δ
...Swift/Authentication/3rd Party/ParseFacebook.swift	100.00% <100.00%> (ø)
Sources/ParseSwift/Coding/ParseCoding.swift	89.36% <100.00%> (+0.23%)	⬆️
Sources/ParseSwift/Storage/ParseFileManager.swift	89.39% <0.00%> (-0.76%)	⬇️
Sources/ParseSwift/Objects/ParseObject.swift	76.77% <0.00%> (-0.28%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 96b26a5...afc29c2. Read the comment docs.