Fix string serialization error raising

[emeryotopalik]

During serialization of the result, String.Chars.to_string raises the following error when given a map:

** (Protocol.UndefinedError) protocol String.Chars not implemented for %{} of type Map. This protocol is implemented for the following type(s): Decimal, Float, DateTime, Time, List, Version.Requirement, Atom, Integer, Version, Date, BitString, NaiveDateTime, URI

This trickles up to the end user when instead it should handle the serialization error more gracefully like the other scalars.

[binaryseed]

Thanks!