Make logs_bloom optional

[fleupold]

Closes #342

More details in issue, but basically mirroring the Block struct from https://github.com/openethereum/openethereum/blob/master/rpc/src/v1/types/block.rs#L48

This required making logs_bloom and total_difficulty optional

Test Plan

I ran the unit tests (but the example block contains only non-null fields). Will also make sure that the example given in the issue now fetches pending blocks successfully.

[tomusdrw]

Would be cool to add a test that contains null (an example pending block)
and fix the CI errors - most likely a cargo fmt run is needed.

[tomusdrw]

Thanks a lot!