extends does not correctly merge maps

Instead of merging maps in order, entire maps are replaced. eg:

job1:
  variables:
    COLOR: red
    SIZE: large

job2:
  variables:
    COLOR: green

job3:
  extends:
    - job1
    - job2

job3 should result in:

job3:
  variables:
    SIZE: large
    COLOR: green

But gitlab-emulator incorrectly produces:

job3:
  variables:
    COLOR: green