Fix mapping for StartDate and EndDate#303

VadimSaratov wants to merge 1 commit intoBingAds:mainBingAds/BingAds-Python-SDK:mainfrom

VadimSaratov:fix/bulk_asset_group_start_end_date_mappingVadimSaratov/BingAds-Python-SDK:fix/bulk_asset_group_start_end_date_mappingCopy head branch name to clipboard