fix(core): explicitly serialise File due to upstream issue

[Malinskiy]

Fixes #552

see google/gson#1875

[codecov-commenter]

Codecov Report

Merging #553 (32e213d) into develop (86b63bb) will decrease coverage by 0.05%.
The diff coverage is 58.33%.

@@              Coverage Diff              @@
##             develop     #553      +/-   ##
=============================================
- Coverage      66.42%   66.37%   -0.06%     
- Complexity       529      533       +4     
=============================================
  Files            120      121       +1     
  Lines           2362     2373      +11     
  Branches         322      323       +1     
=============================================
+ Hits            1569     1575       +6     
- Misses           617      622       +5     
  Partials         176      176              

Impacted Files	Coverage Δ
...tlin/com/malinskiy/marathon/json/FileSerializer.kt	0.00% <0.00%> (ø)
...c/main/kotlin/com/malinskiy/marathon/di/Modules.kt	52.38% <25.00%> (-8.74%)	⬇️
...src/main/kotlin/com/malinskiy/marathon/Marathon.kt	73.91% <100.00%> (+1.82%)	⬆️

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