support unicode strings in column list for copy_from()

It would be a nice feature if the copy_from() method supported unicode strings. For example:

import psycopg2

conn = psycopg2.connect("host=192.168.0.33 user=chris password=postgres dbname=certified_mail_ccc")
cursor = conn.cursor()

cursor.execute("drop table if exists foo")
cursor.execute("create table foo (a varchar(100), b varchar(100))")

import StringIO
source = StringIO.StringIO()
source.write("foo\tbar")
source.seek(0)

# this does not work
#cursor.copy_from(source, "foo", null="None", columns=[u"a", u"b"])

# this does work
#cursor.copy_from(source, "foo", null="None", columns=["a", "b"])