Fix/deeper composite fk

[homersimpsons]

Fixes #207

This fixes a corner case in serialization code about a schema looking like:

  summary_tdh  |  summary     |  entity
  entity_id   <=> entity_id  <=> entity_id
  summary_id  <=> summary_id

Where <=> reflects a Foreign Key

[coveralls]

Coverage increased (+0.006%) to 97.187% when pulling 9d060fc on homersimpsons:fix/deeper_composite_fk into a20710b on thecodingmachine:5.1.

[moufmouf]

There seems to be a PHPStan issue to solve and this should be all right!

[moufmouf]

👍

[moufmouf]

@homersimpsons Can you backport this to the master branch?