Refrain from canonicalizing a client_encoding setting of "UNICODE".

While "UTF8" is the correct name for this encoding, existing JDBC drivers
expect that if they send "UNICODE" it will read back the same way; they
fail with an opaque "Protocol error" complaint if not.  This will be fixed
in the 9.1 drivers, but until older drivers are no longer in use in the
wild, we'd better leave "UNICODE" alone.  Continue to canonicalize all
other inputs.  Per report from Steve Singer and subsequent discussion.