Accessing a flag in a map that has not been set raises an ArgumentError [JIRA: CLIENTS-49]

`map.flags['not_set']` raises an ArgumentError while registers, counters and sets have default values.

I think this happens because https://github.com/basho/riak-ruby-client/blob/master/lib/riak/crdt/inner_flag.rb#L9 doesn't accept a default value. This could be fixed by using `value = nil`.