Do not modify EXIF of original image instance in exif_transpose()

[radarhere]

Resolves #5546

The instance points out that in exif_transpose(),

exif = image.getexif()
...
del exif[0x0112]
transposed_image.info["exif"] = exif.tobytes()

we're deleting the orientation data from the original image's EXIF object (and then, as intended, saving it to the new image instance).

Instead, this PR changes it to

exif = image.getexif()
...
transposed_exif = transposed_image.getexif()
del transposed_exif[0x0112]
transposed_image.info["exif"] = transposed_exif.tobytes()

so that we are deleting the orientation data from the new image's EXIF object only, leaving the original image unaffected.