Fix to_hex_if_need function on string utils

This fix issue #1535. Solution was the same suggested on #1523.

src/utils/string.cc

@@ -136,7 +136,7 @@ std::string toHexIfNeeded(const std::string &str) {
     std::stringstream res;
 
     for (int i = 0; i < str.size(); i++) {
-        int c = str.at(i);
+        int c = (unsigned char)str.at(i);
         if (c < 32 || c > 126) {
             res << "\\x" << std::setw(2) << std::setfill('0') << std::hex << c;
         } else {