Fix deduplication of different errors

[ertrzyiks]

The default value field of the object returned by createErrorObject is an empty string. Having two errors of different type or message share the value and comparison by value alone "" === "" is enough to merge two different errors into one.

createErrorObject(node, "new-kind", "Message 1")
createErrorObject(node, "new-kind", "Message 2")

With this change only errors with the same type, message and value are merged into one.