Fix base64 encoding when char is signed (#279)

**Public-Facing Changes**
Fixed a base64 encoding issue in the C++ example_server.


**Description**
Fixes #278

cpp/examples/example_server.cpp

@@ -18,22 +18,25 @@ static uint64_t nanosecondsSinceEpoch() {
 
 // Adapted from:
 // https://gist.github.com/tomykaira/f0fd86b6c73063283afe550bc5d77594
-// https://github.com/protocolbuffers/protobuf/blob/01fe22219a0312b178a265e75fe35422ea6afbb1/src/google/protobuf/compiler/csharp/csharp_helpers.cc
-static std::string Base64Encode(std::string_view data) {
+// https://github.com/protocolbuffers/protobuf/blob/01fe22219a0312b178a265e75fe35422ea6afbb1/src/google/protobuf/compiler/csharp/csharp_helpers.cc#L346
+static std::string Base64Encode(std::string_view input) {
   constexpr const char ALPHABET[] =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
   std::string result;
   // Every 3 bytes of data yields 4 bytes of output
-  result.reserve((data.size() + (3 - 1 /* round up */)) / 3 * 4);
+  result.reserve((input.size() + (3 - 1 /* round up */)) / 3 * 4);
+
+  // Unsigned values are required for bit-shifts below to work properly
+  const unsigned char* data = reinterpret_cast<const unsigned char*>(input.data());
 
   size_t i = 0;
-  for (; i + 2 < data.size(); i += 3) {
+  for (; i + 2 < input.size(); i += 3) {
     result.push_back(ALPHABET[data[i] >> 2]);
     result.push_back(ALPHABET[((data[i] & 0b11) << 4) | (data[i + 1] >> 4)]);
     result.push_back(ALPHABET[((data[i + 1] & 0b1111) << 2) | (data[i + 2] >> 6)]);
     result.push_back(ALPHABET[data[i + 2] & 0b111111]);
   }
-  switch (data.size() - i) {
+  switch (input.size() - i) {
     case 2:
       result.push_back(ALPHABET[data[i] >> 2]);
       result.push_back(ALPHABET[((data[i] & 0b11) << 4) | (data[i + 1] >> 4)]);