Timezone info is not correctly passed to libexiv2 when writing IPTC and XMP tags

(Originally reported by e-mail by Petri Damstén, thanks Petri!)

For datetime.tzinfo subclasses that are not pyexiv2.utils.FixedOffset, writing a datetime to a tag with a non-null timezone info will result in a wrongly formatted datetime, which cannot be read again subsequently. This affects IPTC tags as well as XMP tags. EXIF tags are not affected as timezone info is discarded (not handled in the specification).

Example script, with any image file (needs python-tz):

import pyexiv2
import pytz
from datetime import datetime

m=pyexiv2.ImageMetadata("image.jpg")
m.read()
m['Xmp.xmp.CreateDate'] = pytz.timezone("US/Eastern").localize(datetime.today())
m.write()
m=pyexiv2.ImageMetadata("image.jpg")
m.read()
print m['Xmp.xmp.CreateDate'].value

Expected result: prints the value of the tag with the correct timezone information.

Current result: "XmpValueError: Invalid value for XMP type [Date]: [2011-06-15T12:47:23.545156EDT]"