Don't overwrite existing ID parameters in default transformation

[nuest]

The documents in my MongoDB schema have an id attribute, which is overwritten by the default transform(..) function. I see different options to make this more flexible (e.g. with an environment variable or by re-introducing settings), but the simplest solution seems to be to only make the "id shift" if the ID parameter does not exist, log a warning, and let the user handle the copying of the id in their transform function.

I am adding changes to that effect to #14

[nuest]

@toystars Are you planning to make a release soon?

[toystars]

@nuest Once there are substantial additions, I'll make a new release.