test_default_encoding_issues():  Fully restore sys.setdefaultencoding.

test_site often failed under "regrtest.py -r", because this xmlrpc test
left sys with a setdefaultencoding attribute, but loading site.py removes
that attribute and test_site.py verifies the attribute is gone.  Changed
this test to get rid of sys.setdefaultencoding if it didn't exist when
this test started.