adding additional log overload to prevent unnecessary fmt::format

[dominicpoeschko]

calling log with a string_view as msg called

 template<typename... Args>
    void log(source_loc loc, level::level_enum lvl, string_view_t fmt, const Args &... args)

instead of

template<class T, typename std::enable_if<std::is_convertible<const T &, spdlog::string_view_t>::value, T>::type * = nullptr>
    void log(source_loc loc, level::level_enum lvl, const T &msg)

which lead to an unnecessary call to fmt::format

[gabime]

Merged. Thanks.