List of headers's keys (separated with ',') whove value are added to the args Header's key lowercased and '-'' replaced to '_' such that 'Content-Type' becomes the 'content_type' arg key
List of headers's keys (separated with ',') whove value are added to the args Header's key lowercased and '-'' replaced to '_' such that 'Content-Type' becomes the 'content_type' arg key