Fixed #14760 - Admin inlines with file/image field fails to save_as

Refactored BaseInlineFormSet and ModelAdmin to copy initial
values (file name) for fields of FileField or it's subclasses.

'physical' file do not get duplicated.

This pull request address following described by ramiro comment (https://code.djangoproject.com/ticket/14760#comment:8):

1. Pressing the "Save as" button reports back a validation error and:

  1.1. The image/file fields of inlines already existing in the DB/FS are empty and show "This field is required." message error.
  1.2. Values of other fields of the already existing inlines are preserved.

It also address "Save as" action in ModelAdmin in the same way.