Redis->new(server=>'') does not connect to localhost:6379 #77

Old logic causes subtle issues in some cases.
Redis->new() remains behaves as used to be, connection to localhost:6379
use exists $args{server} in missing places, and do the same with sock
Merge pull request #78 from PerlRedis/ikruglov/77_new_server_empty

Redis->new(server=>'') does not connect to localhost:6379 #77