squash internal __root__ models in .dict()

[patrickkwang]

Change Summary

This change squashes {'__root__': X} → X when converting a nested model to dict. This does not make that change at the outermost level, so .dict() still always returns a dict (as discussed in #1193).

Related issue number

#1414

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[patrickkwang]

This solution is an alternative to #1429 by @silversurfer34, which is a bit more involved and only impacts .json().

[codecov]

Codecov Report

Merging #1607 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1607   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3796      3796           
  Branches       752       752           
=========================================
  Hits          3796      3796           

---

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 5e82689...bb22ec7. Read the comment docs.