cpp: Fix convert.enum_member test (issue #50).

The patch add serialization operator for the given enum to avoid the use
of a cast which leads to strict-aliasing warnings and the test to fail.

Solution suggested by qehgt.

src/msgpack/type/define.hpp.erb

@@ -34,6 +34,25 @@
 		msgpack::type::make_define(__VA_ARGS__).msgpack_object(o, z); \
 	}
 
+// MSGPACK_ADD_ENUM must be used in the global namespace.
+#define MSGPACK_ADD_ENUM(enum) \
+  namespace msgpack { \
+    template <> \
+    inline enum& operator>> (object o, enum& v) \
+    { \
+      int tmp; \
+      o >> tmp; \
+      v = static_cast<enum>(tmp); \
+      return v; \
+    } \
+    template <> \
+    void operator<< (object::with_zone& o, const enum& v) \
+    { \
+      int tmp = static_cast<enum>(v); \
+      o << tmp; \
+    } \
+  }
+
 namespace msgpack {
 namespace type {