Refactor populateRelation() to separate ArrayAccess implementation

[Tigrov]

Q	A
Is bugfix?	✔️/❌
New feature?	❌
Breaks BC?	❌
Fixed issues	#308

[codecov]

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 87.44%. Comparing base (3beda50) to head (abea1b2).

Files	Patch %	Lines
src/ActiveRelationTrait.php	55.55%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #351   +/-   ##
=========================================
  Coverage     87.43%   87.44%           
  Complexity      601      601           
=========================================
  Files            13       13           
  Lines          1385     1386    +1     
=========================================
+ Hits           1211     1212    +1     
  Misses          174      174           

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