fix/minor: Error encoding hexa decimal

String is defined as an array of char. The char can be negative. The
cast "reinterpret_cast" from char to int keep the negative side, so
the "unsigned char" number 0x91 is negative as "char". When it is
"reinterpret_cast" as integer, it becomes 0xffffff91, so the hexadecimal
display is broken:

   [155493246391.747672] [/absolute?what=badarg2] [9]  T (0) t:hexEncode: "ffffff91ffffffecffffffe6334bffffffebffffff87ffffff9affffff824a06ffffffc33b4cffff (14 characters omitted)"

This patch fix this behavior using classic cast without reinterpret_cast:

   [155493251286.221115] [/absolute?what=badarg2] [9]  T (0) t:hexEncode: "91ece6334beb879a824a06c33b4cb4240e4c6f56"

Author: Thierry Fournier

src/actions/transformations/hex_encode.cc

@@ -41,7 +41,7 @@ std::string HexEncode::evaluate(std::string value,
 
     std::stringstream result;
     for (std::size_t i=0; i < value.length(); i++) {
-        int ii = reinterpret_cast<char>(value[i]);
+        unsigned int ii = (unsigned char)(value[i]);
         result << std::setw(2) << std::setfill('0') << std::hex << ii;
     }