eth/client: fix get balance for block number

[dapi]

Add availability to request eth_get_balance for the block number

[wafcio]

I have prepared this fix: https://github.com/q9f/eth.rb/pull/268. It isn't as easy as yours, but my approach allows us to use eth_call. We should probably support a custom block number for more calls than eth_get_balance and eth_call.

[q9f]

Thanks, I opted for #268