The MongoDB behaviour is to consider a string and a binary different even if they contain the same data, but in a Realm Set<Mixed> they are considered the same. Realm should align to the MongoDB behaviour. In practice this should be a rare case because most strings have a null byte at the end while binaries do not.

Set<Mixed> set;
set.insert(StringData("abc", 3));
set.insert(BinaryData("abc", 3));
REALM_ASSERT(set.size() == 2); // currently fails with size of 1

This change will require a file format bump, but no data migration will be needed.