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/expected/plpython_unicode.out

@@ -1,6 +1,7 @@
 --
 -- Unicode handling
 --
+SET client_encoding TO UTF8;
 CREATE TABLE unicode_test (
 	testvalue  text NOT NULL
 );