scalar wrongly captured by dictionary

Another on the theme of: output keys in the wrong order changing the meaning of the dictionary:
```python
#!/usr/bin/env python3

import tomlkit


doc = tomlkit.loads("a.b = 1\n")
doc["a"]["c"] = {}
doc["z"] = 2

print(tomlkit.dumps(doc))
```
result
```
a.b = 1

[a.c]
z = 2
```
in which `z` is no longer a top-level key