Incorrect RPC response when eth_call fails

[area]

Expected Behavior

When an eth_call is made, and the call fails, a valid JSON response should be received, with the default ('empty') values for the return types in the result field. with a result field of 0x.

Current Behavior

Currently, the JSON response is of the form:

{"id":284,"jsonrpc":"2.0"}

This is missing the result field, as defined in the spec, and so web3 rejects it with Error: Invalid JSON RPC response.

Possible Solution

I think this is because result is undefined when the response is serialized, and gets removed entirely. I don't think returning undefined would be appropriate either though.

Steps to Reproduce (for bugs)

Deploy a contract with the below function, and run eth_call against it:

    function someFunction() public view returns (bool){
        require(false);
        return true;
    }

Your Environment

EthereumJS TestRPC v6.0.3 (ganache-core: 2.0.2) on OSX 10.13.2, Node v8.4.0

[area]

In this scenario, Parity responds with 0x in the response field. Introducing something like

	if (newResponse.result === undefined) {
		newResponse.result = '0x';
	}

to reportErrorInResponse appears to fix the issue for me.