Add a couple tests and prefer underlying for consistency

[travisbrown]

No description provided.

[codecov-io]

Codecov Report

Merging #689 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #689   +/-   ##
=======================================
  Coverage   86.09%   86.09%           
=======================================
  Files          71       71           
  Lines        2187     2187           
  Branches       85       85           
=======================================
  Hits         1883     1883           
  Misses        304      304

Impacted Files	Coverage Δ
...d/src/main/scala/io/circe/NonEmptySeqDecoder.scala	91.66% <0%> (-8.34%)	⬇️
.../core/shared/src/main/scala/io/circe/Decoder.scala	90.65% <0%> (-0.35%)	⬇️
...rc/main/scala/io/circe/shapes/SizedInstances.scala	92.85% <0%> (+14.28%)	⬆️

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