Fix send id

[jadbox]

When using sendAsync, the payload parameter may contain an existing object id. I've added a check for if the id is set and to use it instead of generating a new id.

[moodysalem]

Why should caller ever be responsible for setting id?

[jadbox]

@moodysalem It's because web3js pub sub likes to generate its own IDs for sendAsync actions so it knows which message is the rpc response. It's a weird use-case, but it's needed.

[federico-bohn]

Here we should check for undefined values, if optionalId value is undefined then id will be undefined too

[federico-bohn]

This fix solve an issue that I have when send rpc messages from my app (inside iframe) to metamask:
Wrong response id "3610003435135761" (expected: "13") in {"jsonrpc":"2.0","id":13,"method":"eth_getBlockByNumber","params":["latest",false]}