Fixed operator< on ZString.

testing/zstring_test.cpp

@@ -77,5 +77,9 @@ int main(int argc, char **argv) {
    coreutils::ZString test10("value1");
    coreutils::ZString test11("value1");
    std::cout << test10.compare(test11) << ":" << test11.compare(test10) << std::endl;
+
+   coreutils::ZString test12("1");
+   coreutils::ZString test13("12");
+   std::cout << (test12 < test13) << ":" << (test13 < test12) << std::endl;   
    
 }