Added -Wconversion support for C++.

test/json.cpp

@@ -1,8 +1,14 @@
 #include <msgpack.hpp>
 #include <fstream>
 #include <sstream>
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wconversion"
+
 #include <gtest/gtest.h>
 
+#pragma GCC diagnostic pop
+
 TEST(json, basic_elements)
 {
     typedef std::map<std::string, int> map_s_i;