return a new instance of Data in decodeSingleType

[zhangliugang]

Summary of Changes

Fixes #801

I got the same issue as @6od9i when i tried to decoding ERC20 balances from a multicall contract.
I think the problem is that when decoding bytes type, a slice of the data is returned, resulting in wrong index. retuning a
new Data instance can fix this.

Test Data or Screenshots

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[6od9i]

Yes, I think this commit can fix my problem too, but both PRs should be merged, because it is could be more safe for future changes.