Override default merge method for our configuration

For config values of type Array, we don't check for UNSET_VALUE
but only if the array is empty or not

Fix #146
Why disable rubocop here ? The style bug can be fixed

```ruby
# Ignore keys that start with a double underscore. This allows
# configuration classes to still hold around internal state
# that isn't propagated.
next if key.to_s.start_with?('@__')

# Don't set the value if it is the unset value, either.
value = obj.instance_variable_get(key)
if [:@networks, :@volumes, :@rsync_includes].include? key
  result.instance_variable_set(key, value) unless value.empty?
else
  result.instance_variable_set(key, value) if value != UNSET_VALUE
end
```