🐛[Bug] [Extension] Fix KotlinX serialization problem

[kittinunf]

Description

This PR fixes #720

Type of change

Check all that apply

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (a change which changes the current internal or external interface)
• This change requires a documentation update

How Has This Been Tested?

In case you did not include tests describe why you and how you have verified the
changes, with instructions so we can reproduce. If you have added comprehensive
tests for your changes, you may omit this section.

Have tested again mock server, and https://httpbin.org/.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation, if necessary
• My changes generate no new compiler warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Inspect the bytecode viewer, including reasoning why

[codecov]

Codecov Report

Merging #733 into master will increase coverage by 0.18%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #733      +/-   ##
============================================
+ Coverage     72.53%   72.71%   +0.18%     
  Complexity      301      301              
============================================
  Files            61       61              
  Lines          1511     1510       -1     
  Branches        205      205              
============================================
+ Hits           1096     1098       +2     
+ Misses          314      311       -3     
  Partials        101      101              

Impacted Files	Coverage Δ	Complexity Δ
.../kittinunf/fuel/serialization/FuelSerialization.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...kittinunf/fuel/core/requests/SuspendableRequest.kt	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...kotlin/com/github/kittinunf/fuel/core/FuelError.kt	86.20% <0.00%> (+0.49%)	14.00% <0.00%> (ø%)
...ub/kittinunf/fuel/core/requests/DownloadRequest.kt	90.00% <0.00%> (+0.52%)	7.00% <0.00%> (ø%)

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[iNoles]

Interesting

[arosenb2]

This looks good to me. Thanks for patching so quick!