Use drupal_set_header for file_transfer()

Other modules would certainly benefit from knowing what kind of headers are currently set. Using drupal_set_header() is identical in functionality other than it keeps a record of the current headers. This one line fix will enable devel module to work with query logging and private file transfers, and will keep it from breaking upload.module, imagefield.module, and others.

Applying to 5.x and 4.7 would be preferable also.