Write the '\0' in the right place when converting exif dates to iso8601
Rename option to replace 'date posted' with 'date taken', to avoid confusion

Changed it from "Set 'Taken Date' as 'Posted Date'" to "Replace 'Date
Posted' with 'Date Taken'", will hopefully be less confusing. Also,
renamed internal variables and functions, as well as the name of the
property in the settings.xml, to reflect the change.