Fix integer warnings

test/benchmarks/buffers.cpp

@@ -71,7 +71,7 @@ public:
             std::fill(
                 buffer_cast<char*>(buffer),
                 buffer_cast<char*>(buffer) +
-                    buffer_size(buffer), 0);
+                    buffer_size(buffer), '\0');
             n += buffer_size(buffer);
         }
         return n;

test/websocket/utf8_checker.cpp

@@ -49,7 +49,7 @@ public:
                 BEAST_EXPECT(! utf8.write(&(*it), 1));
 
         // Invalid sequence
-        std::fill(buf.begin(), buf.end(), 0xFF);
+        std::fill(buf.begin(), buf.end(), '\xff');
         BEAST_EXPECT(! utf8.write(&buf.front(), buf.size()));
     }