Support #scan with type option

[bertm13]

👋 This PR adds type option support to scan and scan_each. See the Redis docs here.

In order to make testing easier, I also added an optional enumerator parameter to the intializer of Zset.
I also altered the scan and scan_each specs to define the data hash instead of collection key array, since the Database#type method requires inspecting the data hash's values.

[sds]

Thanks!