Incorrect merge of service's environment when using map

[dustinrc]

Given the two files:

# docker-compose.yml
version: '2'
services:
  simple:
    image: busybox
    environment:
      alpha: one
      bravo: two
      zulu: twenty-six
    command: env

and

# docker-compose.override.yml
version: '2'
services:
  simple:
    environment:
      charlie: three
      bravo: four

the expected result is an environment with alpha=one, bravo=four, charlie=three and zulu=twenty-six. This is the outcome when using docker-compose, but with libcompose the environment only includes bravo=four and charlie=three.

If the second file defines environment as a list of strings:

# docker-compose.override.yml
version: '2'
services:
  simple:
    environment:
      - charlie=three
      - bravo=four

then the merge is correct.