* feat: support serialization of array, span and unordered_map
* fix some spelling mistakes
* docs: update table of valid c++ types
---------
Co-authored-by: Marcel Hellwig <github@cookiesoft.de>
* Add missing nullptr check in Message::peekType().
* According to its specification, sd_bus_message_peek_type() sets a
given contents parameter to NULL if the next element in a message is
not a container. Since assigning a nullptr to a std::string has
undefined behaviour (typically resulting in an invalid memory access),
Message::peekType() must not assign contentsSig unconditionally.
