[Test] ManyToMany uncasts wrong fields

[roxblnfk]

Test case for #427

[codecov[bot]]

Codecov Report

Merging #428 (f0b7197) into 2.x (8b85b83) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x     #428      +/-   ##
============================================
+ Coverage     91.38%   91.40%   +0.02%     
  Complexity     1760     1760              
============================================
  Files           112      112              
  Lines          4480     4480              
============================================
+ Hits           4094     4095       +1     
+ Misses          386      385       -1     

Files Changed	Coverage Δ
src/Relation/ManyToMany.php	95.94% <100.00%> (ø)

... and 1 file with indirect coverage changes