Empty key-value pair in a list is not dumped correctly

[vlandeiro]

Using the latest commit on master (3f637db at this time), here is a minimal example to reproduce the issue I'm encountering:

>>> import toml
>>> broken = {"foo": [{"bar": {}, "moot": {"a": 1}}]}
>>> broken
{'foo': [{'bar': {}, 'moot': {'a': 1}}]}
>>> print(toml.dumps(broken))
[[foo]]

[foo.moot]
a = 1
>>> toml.loads(toml.dumps(broken))
{'foo': [{'moot': {'a': 1}}]}
>>> toml.loads(toml.dumps(broken)) == broken
False

Note how the bar key disappears when dumping the dictionary to TOML, leading to data loss when passing the original data through a dumps/loads round trip.

The expected behavior is that toml.dumps(broken) returns the following TOML that can then be parsed to retrieve the original data.

[[foo]]
[foo.bar]

[foo.moot]
a = 1

Another thing to note is that this is only an issue when the key-value pair is part of a list. For instance, the following example behaves as expected:

>>> working = {"foo": {"bar": {}}}
>>> toml.loads(toml.dumps(working))
{'foo': {'bar': {}}}
>>> toml.loads(toml.dumps(working)) == working
True