Add type param to autoUnwrap to improve type inference

[fthomas]

closes #608

[fthomas]

I verified that the added test fails to compile prior to this change.

[codecov]

Codecov Report

Merging #612 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #612   +/-   ##
=======================================
  Coverage   93.15%   93.15%           
=======================================
  Files          65       65           
  Lines         716      716           
  Branches       10       10           
=======================================
  Hits          667      667           
  Misses         49       49

Impacted Files	Coverage Δ
...hared/src/main/scala/eu/timepit/refined/auto.scala	100% <ø> (ø)	⬆️

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