Setting the expiry through the setter instead of the constructor for KV mutations results in an ArgumentError

Utils::Time.extract_expiry_time(expiry) transforms the expiry to the format expected by the wrapper C++ backend. Currently it is called in the constructor, which means that if expiry is set through the setter, it is not being called and the expiry is sent to the backend in the incorrect format resulting in an ArgumentError.
 
To resolve this, extract_expiry_time should be called in the #to_backend method instead