Generic encoding incorrect for some cases

Consider a type like this:

```
data T = A { tag :: Int } | B
```

when using the default options for generic encoding and try to encode `A { tag = 2 }` the result is `{"tag":2}`, because the field `tag` overwrote the actual constructor tag `A`

Start by reproducing the generic encoding example for `data T = A { tag :: Int } | B` with the default options and inspect the generic encoding entry points. Done means the constructor tag `A` is not overwritten by the field named `tag`, with behavior confirmed by a regression test.

Written by the indexing model from the issue text.