Set client encoding explicitly in plpython_unicode test

This will (hopefully) eliminate the need for the
plpython_unicode_0.out expected file.

src/pl/plpython/sql/plpython_unicode.sql

@@ -2,6 +2,8 @@
 -- Unicode handling
 --
 
+SET client_encoding TO UTF8;
+
 CREATE TABLE unicode_test (
 	testvalue  text NOT NULL
 );