fix has one relation update

[patricklx]

fixes #572
fixes #535

[codecov]

Codecov Report

Merging #575 (9003dfd) into master (0e2b07f) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #575      +/-   ##
==========================================
- Coverage   95.77%   95.73%   -0.04%     
==========================================
  Files          21       21              
  Lines        2509     2509              
==========================================
- Hits         2403     2402       -1     
- Misses        106      107       +1     

Impacted Files	Coverage Δ
src/Relation/HasOne.js	87.50% <100.00%> (ø)
src/Relation.js	98.07% <0.00%> (-0.97%)	⬇️

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