Fix email fields not being loopable

[jasonvarga]

Fixes #5360

This PR will reindex the collection so it'll work across parsers.

---

Turns out the old parser allowed looping over non-numerically-indexed collections, but only if they were a collection. Arrays didn't work. We never documented it as working though.

Numerically-indexed array:

field:
  - # technically 0
    title: Alfa
  - # technically 1
    title: Bravo

{{ field }} {{ title }} {{ /field }} // Works fine. Outputs: Alfa Bravo

Non-numerically indexed

field:
  foo:
    title: Alfa
  bar:
    title: Bravo

{{ field }} {{ title }} {{ /field }} // This shouldn't work, but was, but only if it was a collection.

I don't consider this a parser bug since it's an undocumented case.