Add optimisation for size encoding for short/int/long and
double
Change encoding for null keys. The number of consecutive null keys is
encoded less 1 to have TYPE_NULL = 0 when we have only a null key. Now
let some many 0 at the end of byte[] is decodable. Useful if you want to
write a Msg with the same size.
Fix bug when length of consecutive null keys is encoded in a int
Fix pb when call set double with 7-8 precision.
Small Refactoring