Fixes error in case of non-default column name

"error undefined method `length' for nil:NilClass"
In case of default :flag_query_mode (:in_list) and non-default :column options,
flag_options[colmn][:column] is a symbol, but flag_mapping has string keys.
Storing column name in flag_options[colmn][:column] is unuseful and it is duplication of colmn.