Invalid serialization after removal of array item #405

>>> from tomlkit import parse
>>> doc = parse('x = [\n    "foo" ,#spam\n    "bar",\n]')
>>> doc['x'].remove('foo')
>>> docstr = doc.as_string()
>>> print(docstr)
x = [,#spam
    "bar",
]
>>> parse(docstr)
Traceback (most recent call last):
  File "<python-input-5>", line 1, in <module>
    parse(docstr)
    ~~~~~^^^^^^^^
  File "W:\tomlkit\tomlkit\api.py", line 91, in parse
    return Parser(string).parse()
           ~~~~~~~~~~~~~~~~~~~~^^
  File "W:\tomlkit\tomlkit\parser.py", line 139, in parse
    item = self._parse_item()
  File "W:\tomlkit\tomlkit\parser.py", line 238, in _parse_item
    return self._parse_key_value(True)
           ~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "W:\tomlkit\tomlkit\parser.py", line 331, in _parse_key_value
    val = self._parse_value()
  File "W:\tomlkit\tomlkit\parser.py", line 435, in _parse_value
    return self._parse_array()
           ~~~~~~~~~~~~~~~~~^^
  File "W:\tomlkit\tomlkit\parser.py", line 608, in _parse_array
    raise self.parse_error(UnexpectedCharError, self._current)
tomlkit.exceptions.UnexpectedCharError: Unexpected character: ',' at line 1 col 5