fix bit padding bugs in bucket_set/unset

2 bugs are fixed by this commit.

1. bit flag is not set correctly when counter increase.
Adding same item cause counter bitmap carry over 1 bit 
but without setting low bit flag. 
This will cause false negative error on `include?` check.

2. carry over bits are lost due to type casing.
Counter may use up-to 14bits as bitmap. But `bf->ptr` is
`unsigned char` type. So the extra 6 bits may lost due to
type casting.