UserSubclass.become should return subclass

[kzmeyao]

This is a follow-up to #756 to add the same functionality to become.

Let me know if there's anything I'm missing.

[codecov]

Codecov Report

Merging #897 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #897   +/-   ##
======================================
  Coverage    92.1%   92.1%           
======================================
  Files          54      54           
  Lines        5026    5026           
  Branches     1127    1127           
======================================
  Hits         4629    4629           
  Misses        397     397

Impacted Files	Coverage Δ
src/ParseUser.js	83.18% <100%> (ø)	⬆️

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[dplewis]

LGTM! Thanks for the PR!