Fix marshal smallint

[illia-li]

Fixed for smallint type:

1. string marshal and unmarshal as nullable.
   1.1. Was: marshals "" - caused an error; unmarshals nil data into "0".
   1.2. Now: marshals and unmarshals nil data - "".
2. custom string marshals and unmarshals was not supported, now supported.
3. big.Int marshals was not supported, now supported.
4. Also was fixed cases when marshal and unmarshal should an error, but currently we does not have a corrupt test suite.

Close issues for smallint:

1. Marshal, unmarshal. For many cql types unmarshaling data that have bigger len than cql type, do not return an error. #246
2. Marshal, unmarshal. For many cql types a go string is supported, but not a go custom string #243
3. Marshal, unmarshal. cql types: tinyint, smallint, int can not be marshaled into go big.Int #244
4. Marshal, unmarshal. Problems with zero and null data and values. #250