since mergo had been bumped to 0.3.9 an environment value like:
```
affinity: {}
```
was not merged properly (not merged at all) instead it threw an error
that it cannot find the key "affinity" in the environment values even
though it was outputted in debug output as read in properly.
Probably since #647 helmfile has been unable to merge nested maps in environment values if they were loaded from files. This fixes it.
The relevant test is also enhanced so that no further regression like this happens.
Fixes#677