Added -Wconversion support for C++.

test/msgpack_tuple.cpp

@@ -1,6 +1,12 @@
 #include <msgpack.hpp>
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wconversion"
+
 #include <gtest/gtest.h>
 
+#pragma GCC diagnostic pop
+
 
 TEST(msgpack_tuple, member_get)
 {