Serialisation with an empty string as key fails #37

>>> json5.dumps({'': 'value'})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/fornwall/src/pyjson5/json5/lib.py", line 232, in dumps
    _, v = _dumps(obj, skipkeys, ensure_ascii, check_circular,
  File "/Users/fornwall/src/pyjson5/json5/lib.py", line 304, in _dumps
    s = _dump_dict(obj, skipkeys, ensure_ascii,
  File "/Users/fornwall/src/pyjson5/json5/lib.py", line 342, in _dump_dict
    valid_key, key_str = _dumps(key, skipkeys, ensure_ascii, check_circular,
  File "/Users/fornwall/src/pyjson5/json5/lib.py", line 253, in _dumps
    if (is_key and _is_ident(obj) and not quote_keys
  File "/Users/fornwall/src/pyjson5/json5/lib.py", line 442, in _is_ident
    if not _is_id_start(k[0]) and k[0] not in (u'$', u'_'):
IndexError: string index out of range

>>> json.dumps({'': 'value'})
'{"": "value"}'