Incorrect string returned by dumps when moving/renaming table

[abravalheri]

Hello @sdispater, thank you again for the amazing library.

I recently noticed that there is a problem with dumps when "moving" an element of an existing parsed document. In the case of renaming an existing table, the "dumped" string keeps the old name. Please check the example bellow:

>>> import tomlkit
>>> example = """\
... ["nested.key"]
... a = 3 # comment
... """
>>> doc = tomlkit.parse(example)
>>> doc["nested"] = {}
>>> doc["nested"]["key"] = doc.pop("nested.key")
>>> doc
{'nested': {'key': {'a': 3}}}

>>> print(tomlkit.dumps(doc))

[nested]
["nested.key"]
a = 3 # comment

>>>

I would expected the "dumped" string to be the following:

[nested]
[nested.key]
a = 3 # comment

Please notice that the tables don't have to be nested, as can be seen in the example bellow:

>>> example = """\
... [x]
... a = 3 # comment
... """
>>> doc = tomlkit.parse(example)
>>> doc["y"] = doc.pop("x")
>>> doc
{'y': {'a': 3}}

>>> print(tomlkit.dumps(doc))

[x]
a = 3 # comment

>>>

I am using Python 3.8.0 and tomlkit 0.7.2 on Ubuntu 18.04.5 LTS