ethclient: fix parity compatibility on newHeads

[Ivshti]

This issue addresses an incompatibility between ethclient and Parity - currently, there is no way to use the newHeads call of the pubsub spec with a Parity node.

It appears that Parity has a stricter argument check on second argument of the eth_subscribe JSON-RPC call.

While ethclient sends a subscription message that looks like:

{"id": 1, "method": "eth_subscribe", "params": ["newHeads", {}]}

Reference:

go-ethereum/ethclient/ethclient.go

Line 299 in 762f3a4

return ec.c.EthSubscribe(ctx, ch, "newHeads", map[string]struct{}{})

Parity rejects it with this error: https://github.com/paritytech/parity/blob/d7a7f034db6e7d84e2182ab2cd76cfc0a438723c/rpc/src/v1/impls/eth_pubsub.rs#L276

Therefore making Geth's ethclient package incompatible with the implementation in Parity

The issue has been discussed here:
openethereum/parity-ethereum#7731

And a fix has been proposed but closed here:
openethereum/parity-ethereum#7732

The fixes are either:

1. Do not send a second argument (empty map) in ethclient
2. Loosen the requirement on parity

To me (1) makes more sense, since the second argument is redundant anyway. It should be noted that this change makes ethclient compatible with Parity without breaking any compatibility with Geth.

Unfortunately, there are no specifics regarding the second argument in https://github.com/ethereum/go-ethereum/wiki/RPC-PUB-SUB

[fjl]

Thank you!