Added clause_pair as native type

libraries/chain/contracts/abi_serializer.cpp

@@ -71,6 +71,7 @@ namespace eosio { namespace chain { namespace contracts {
       built_in_types.emplace("checksum160",               pack_unpack<checksum160_type>());
       built_in_types.emplace("checksum256",               pack_unpack<checksum256_type>());
       built_in_types.emplace("checksum512",               pack_unpack<checksum512_type>());
+      built_in_types.emplace("clause_pair",               pack_unpack<clause_pair>());
       built_in_types.emplace("transaction_id_type",       pack_unpack<checksum256_type>());
       built_in_types.emplace("field_name",                pack_unpack<field_name>());
       built_in_types.emplace("fixed_string32",            pack_unpack<fixed_string32>());