table escapes its parent

```python
#!/usr/bin/env python3

import tomlkit

doc = tomlkit.loads("""
[t]
a.b = 1
""")
doc["t"]["a"]["c"] = {}

output = tomlkit.dumps(doc)

print(output)
```
result
```
[t]
a.b = 1

[a.c]
```
in which `[a.c]` is not a child of `t` as it should be