fix signed/unsigned warning

tests/values.cpp

@@ -123,8 +123,8 @@ BOOST_AUTO_TEST_CASE(position_ops)
 	v.push_back(minus_one);
 	v.push_back(minus_two);
 	
-	for(int i = 0; i < v.size(); ++i) {
-		for(int j = 0; j < v.size(); ++j) {
+	for(size_t i = 0; i < v.size(); ++i) {
+		for(size_t j = 0; j < v.size(); ++j) {
 			BOOST_CHECK_EQUAL(i == j, v[i].equals(v[j]));
 			BOOST_CHECK_EQUAL(i == j, v[j].equals(v[i]));
 			if(i >= 2 && j >= 2) { //nowhere, zero, one, minus_one, minus_two