copy_field/move_field to existing field does not overwrite but create an object

[TobiasNx]

When using copy_field/move_field to an existing field.
Fix does not overwrite but create an object with multiple instances. I would have expected that the field would be overwritten.

See here

Output is:

{
  "animal" : {
    "1" : "dog",
    "2" : "cat"
  },
  "bnimal" : "cat"
}

Output expected:

{
  "animal" : "cat",
  "bnimal" : "cat"
}

[TobiasNx]

Perhaps this should also be true for add_field

[fsteeg]

Duplicate of #116, closing.