Fix FromSql/ToSql for serde_json::Value

[gwenn]

Fix #882 for valid JSON values

[codecov]

Codecov Report

Base: 65.62% // Head: 65.64% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (6ca5432) compared to base (12a6d3c).
Patch coverage: 82.35% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1252      +/-   ##
==========================================
+ Coverage   65.62%   65.64%   +0.02%     
==========================================
  Files          45       45              
  Lines        3785     3799      +14     
==========================================
+ Hits         2484     2494      +10     
- Misses       1301     1305       +4     

Impacted Files	Coverage Δ
src/types/serde_json.rs	84.21% <82.35%> (-15.79%)	⬇️
src/types/value_ref.rs	49.01% <0.00%> (-2.95%)	⬇️
src/error.rs	23.28% <0.00%> (+1.36%)	⬆️

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