Writer::WriteUint: add cast to hide warning C4244 on MSVC

include/rapidjson/writer.h

@@ -185,7 +185,7 @@ protected:
 		char buffer[10];
 		char *p = buffer;
 		do {
-			*p++ = (u % 10) + '0';
+			*p++ = char(u % 10) + '0';
 			u /= 10;
 		} while (u > 0);