signTypedData_v4 not according to specification

[cammellos]

I was implementing signTypedData_v4 for our project, and I have used the tests in this repo to make sure our implementation was sound.

I then noticed that geth provided their own https://github.com/ethereum/go-ethereum/blob/43c278cdf93d5469702fd1c2f570dbf3c1718ff0/signer/core/signed_data.go#L323 , so I plugged in the tests from this repo in geth and noticed that the behavior was not consistent.

Upon further investigation, I noticed that currently the behavior of signTypedData_v4 is not according to
https://eips.ethereum.org/EIPS/eip-712 when it comes to encoding arrays.

The eip states:

The array values are encoded as the keccak256 hash of the
concatenated encodeData of their contents

The behavior instead was to encode array values as the keccak256 of the
concatenated keccak256 of the values.

This worked well for primary types, but not for struct, as encodeData
per spec is:

The encoding of a struct instance is enc(value₁) ‖ enc(value₂) ‖ … ‖ enc(valueₙ) , i.e. the concatenation of the
encoded member values in the order that they appear in the type.
Each encoded member value is exactly 32-byte long.

Instead, we are using basically hashStruct instead of encodeData