Use creation information from asset case class for modification information if specified for asset
Leave modification info empty when creation time and modification time is unspecified
Use modification information if both modification and creation information are specified
Remove unused property handling code
Use creation time from case class instead of from properties in tests
Specify common functionality of extracting asset validity period in shared function
Reduce visibility of helper function
Merge pull request #218 from finnishtransportagency/use-modification-and-creation-data-from-case-class-in-csv-export

Use modification and creation data from case class in csv export