Fix size packing/unpacking for EXT 8/16/32

For EXT 8/16/32, the "size" field was being incremented by 1 to account for the
type field, but according to the specification the size should only consider the
length of the data field.
Merge pull request #129 from tarruda/fix-ext-8-16-32

Fix size packing/unpacking for EXT 8/16/32