Reuse RFC3339DateFormatter when encoding/decoding a JSONFeed

[sebj]

Three small changes relating to date formatters, each in a separate commit:

• Reuse RFC3339DateFormatter when encoding or decoding a JSONFeed
  • Totally open to suggestions on the formatting/style of this to fit the project
• Make date formatter classes final – seemed like an optimization to be made, given it's unlikely these will be subclassed
• Added a mention of RFC 1123 to the documentation comment of FeedDateFormatter, which currently supports multiple specs

[nmdias]

Hi @sebj

Looks great 😄 Reusing the formatter makes sense and making others final is a nice optimization.

swift format should take care of any formatting/style 🚀

Thank you so much.

[nmdias]

@sebj just something I noticed now.

I created a branch reuse-date-formatters to test out moving all formatters to a shared instance, but an issue may came up. In batch processing feeds a shared instance is mutated by multiple threads. The dateFormat changes as it attempts to parse multiple date formats. JSON feeds aren't so popular, so the issue is very unlikely to present itself there.

[sebj]

Ah right, sorry I didn't quite realise that when making my initial change here – that's a little annoying

[nmdias]

All is good, I kept everything else on the main branch and rc.6

Thank you 😄