fix HeaderExtractor when using accessor in ActiveRecord

This should fix comma-csv/comma#85
Merge pull request #86 from eitoball/fix_header_extractor_with_accessor_in_active_record

Fix HeaderExtractor when using accessor in ActiveRecord
Fixed a bug which occurs when a polymorphic association has a hash parameter specified
add spec for adad91ac917462a363be5773bd38dbaa2f31ed27