Problem: `sendToCosmos` can be called with non erc20 tokens

[yihuang]

https://github.com/FrancoCRO/eth-gravity/blob/feature/erc721-to-gravity/test/Gravity.test.js#L19

Franco did the test that one can call sendToCosmos with an erc721 token, because erc721 and erc20 using the same method signatures, but gravity bridge will treat it as a fungible token and cause a disastrous result, user could lose their nfts.

[yihuang]

https://github.com/yihuang/cronos/commit/34bff5907f8383638081c9952fc2fe8f79543d3b This is one solution I came up with, not sure if there is a more generic solution.