Support list and other models with gsonPref

[wrozwad]

fixes #87

[chibatching]

Hi, thank you for your PR!
It looks good for me, but CI failed because compile error.
Can you fix it?

[codecov]

Codecov Report

Merging #88 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   59.38%   59.38%           
=======================================
  Files          23       23           
  Lines         261      261           
  Branches       57       57           
=======================================
  Hits          155      155           
  Misses         60       60           
  Partials       46       46

Impacted Files	Coverage Δ
...batching/kotpref/gsonpref/KotprefGsonExtentions.kt	33.33% <0%> (ø)	⬆️
...m/chibatching/kotpref/gsonpref/GsonNullablePref.kt	53.84% <100%> (ø)	⬆️
...otlin/com/chibatching/kotpref/gsonpref/GsonPref.kt	50% <100%> (ø)	⬆️

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[wrozwad]

So... I added missing imports but it's not my fault that it doesn't hit 50% diff target 😛