as_string(), dumps() incorrectly escape table name "\\"

[andreareina]

The .as_string() should return '["\\\\"]' not '["\\"]', i.e. the on-file document should look like

["\\"]

not

["\"]

reproduction script:

import tomlkit

tomlkit.__version__             # '0.11.5'
doc = tomlkit.TOMLDocument()
doc.add("\\", tomlkit.table())  # {'\\': {}}
doc.as_string()                 # '["\\"]\n'
print(doc.as_string())
tomlkit.loads(doc.as_string())  # error!
tomlkit.loads('["\\\\"]\n')     # {'\\': {}}

interpreter trace:

>>> tomlkit.__version__
'0.11.5'
>>> doc = tomlkit.TOMLDocument()
>>> doc.add("\\", tomlkit.table())
{'\\': {}}
>>> doc.as_string()
'["\\"]\n'
>>> print(doc.as_string())
["\"]

>>> tomlkit.loads(doc.as_string())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "[...]/tomlkit-repro.py", line 7, in <module>
  File "[...]/.venv/lib/python3.8/site-packages/tomlkit/api.py", line 42, in loads
    return parse(string)
  File "[...]/.venv/lib/python3.8/site-packages/tomlkit/api.py", line 82, in parse
    return Parser(string).parse()
  File "[...]/.venv/lib/python3.8/site-packages/tomlkit/parser.py", line 158, in parse
    key, value = self._parse_table()
  File "[...]/.venv/lib/python3.8/site-packages/tomlkit/parser.py", line 906, in _parse_table
    key = self._parse_key()
  File "[...]/.venv/lib/python3.8/site-packages/tomlkit/parser.py", line 362, in _parse_key
    return self._parse_quoted_key()
  File "[...]/.venv/lib/python3.8/site-packages/tomlkit/parser.py", line 378, in _parse_quoted_key
    key_str = self._parse_string(
  File "[...]/.venv/lib/python3.8/site-packages/tomlkit/parser.py", line 825, in _parse_string
    raise self.parse_error(InvalidControlChar, code, "strings")
tomlkit.exceptions.InvalidControlChar: Control characters (codes less than 0x1f and 0x7f) are not allowed in strings, use \u000a instead at line 1 col 5
>>> tomlkit.loads('["\\\\"]\n')
{'\\': {}}