If a values associated type `Serializable` , equals the values type, `toValue` would infinitely call itself.

The test `testWrongCast` reproduces this and I fixed it by comparing the `nextType` with the current type.

In my app I have a use case, where I have a dynamic array of keys, which I want to observe using a Publisher (a publisher works better here then AsyncSequence). By giving an option to use either variadic parameters or an array, that would be possible without breaking any code.

Additionally, I've discovered a crash when observing multiple keys and immediately cancelling the publisher. Here an observer, which was not yet added, was removed. Added check to make sure, this can't happen.

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>