Fixing encoding of decimals ending in .0 or 0.0

Found an error where storing a BigDecimal ending in .0 or 00.0 would not store/retrieve correctly.
Traced this to the append_decimal method of CqlByteBuffer

Example: 
Encoding a BigDecimal of 1042342234234.0 would result in a decoded decimal of -57169393542.0
Encoding a BigDecimal of 12000.0 was unable to be decoded