fix: object merging

[cossssmin]

Switched to using a custom function based on lodash.mergeWith instead of lodash.merge, to avoid overwriting array values when merging objects (reference). Fixes #1057.

Given this:

merge(
  {foo: ["a", "b", "c"]},
  {foo: ["d"]}
)

Previously:

{
  foo: ["d", "b", "c"]
}

Now:

{
  foo: ["a", "b", "c", "d"]
}