Add correct DateTime (editing date) to Exif data

Currently, RT does not save the editing date (Exif tag DateTime) in the exif data of the exported files, only DateTimeOriginal (the date/time the photo was captured). This patch fixes this.

rtengine/metadata.cc

@@ -317,7 +317,7 @@ void Exiv2Metadata::saveToImage(const Glib::ustring &path, bool preserve_all_tag
     std::time_t t = std::time(nullptr);
     char mbstr[100];
     if (std::strftime(mbstr, sizeof(mbstr), "%Y:%m:%d %H:%M:%S", std::localtime(&t))) {
-    dst->exifData()["Exif.Image.ModifyDate"] = mbstr;
+    dst->exifData()["Exif.Image.DateTime"] = mbstr;
     }
     
     import_exif_pairs(dst->exifData());