Handle unexpected tokens in mapping_util

[GGoetzelmann]

jsonpath-ng struggles to properly handled some tokens such as # in the dotted path.
The problem can be addressed by putting fields in ''.
However, putting fields in brackets breaks treatment for lists such as .fieldname[] or .fieldname[0].
At the moment such fields are excluded from escaping, but that would mean that a combination such as .#fieldname[] would break the parsing.

Ideally we find a way to do this more consistently and thoroughly