Reading a tag off an image unnecessarily re-writes its value(s)

This is mostly a code issue, possibly leading to poor performances when reading a tag off an image.
When a tag is read off an image, pyexiv2's internal machinery calls the corresponding static method called {Exif,Iptc,Xmp}Tag._from_existing_tag(...).

Example code path for an IPTC tag:
This sets the raw values of the newly created tag, which calls self._set_raw_values(...), which in turn calls self._tag._setRawValues(...), resetting the raw values of the tag, which is totally unnecessary, and potentially impairs the performances.