fix: Update the JSON-RPC to correct the return information.

[WilliamXieCrypto]

Fixes: #632

Solution:

Before the fix, the "web3_sha3" JSON_RPC endpoint will overwrite the "0x" as "" by using "hexutil.Bytes" as the input parameters, then return same result. After the fix, using "string" as the input parameters will avoid the overwrite and return correct response.

Description

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[fedekunze]

Thanks! minor comment to move the tests to the new package