Dot in key is not parsed correclty on loads()

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Hey, devs!
First, thanks for the great project! Really useful and helpful.
I found a weird behavior while experimenting with dictionary keys containing dots (".") in it. Here is the example:
```
>>> from marshmallow import Schema, fields
>>> TestSchema = Schema.from_dict({"something": fields.Str(), "some.thing": fields.Str()})
>>> TestSchema().loads('{"something": "data"}')
{'something': 'data'}
>>> TestSchema().loads('{"some.thing": "data"}')
{'some': {'thing': 'data'}}
```
Is it a correct behavior? How to work around it?

Reproduce the two Schema.from_dict(...).loads(...) examples first, then trace how loads() handles dotted keys. Because no target behavior, file, or test is named, confirm the intended handling with maintainers; done should be an agreed behavior with regression coverage and a documented workaround if needed.

Written by the indexing model from the issue text.