Fix return value of seestar_cqltypes:decode_map/5

Seestar expects following structure for encoding as value for Cassandra map:
dict:from_list([{Key1, Value1}, {Key2, Value2}, ...]).
When decoding Cassandra map it returns this structure:
dict:from_list([{Key1, [Value1]}, {Key2, [Value2]}, ...]).
Goal of this fix is to make seestar accept same value for encoding, as
it returns from decoding.
Merge pull request #1 from janchochol/decode_map

Fix seestar_cqltypes:decode_map/5