Issue title: Invalid JSON output
 Issue body: The JSON output for the chain_getBlock function seems invalid.
I believe arrays are incorrectly enclosed by double quotes.
i.e. removing the double quotes before { and after }.
 Comments: 
 Comment 0: Those are stock-standard `Uint8Array`. However, when `JSON.stringify(<object>)` is called on something containing it, it returns the less-than-useful output.

I built a small hack to make sense of both BN & Uint8Array, e.g.

https://github.com/polkadot-js/apps/blob/a6f6d1aefdf47f203e9e5e58f61e5ff49a9da668/packages/app-explorer/src/BlockByHash/BlockByHash.tsx#L29

This is used for debugging purposes, i.e. go to https://poc-2.polkadot.io/#/explorer/hash/0x12cbd85cb473172e6cb4c15ab6862ae236cffb0098fc479044f530b74885fe75 and look at the console output, 
 Comment 1: https://gist.github.com/emielvanderhoek/28c5b8987ff340ac4d0959e0e6a48471
 Comment 2: Ahhh, the rpc page - yes, that decoding does not do any magic atm. Would be simple to run it through the same type of logic there. Tagging as such.
 Comment 3: This is the offending area (seemingly very complicated) -

 https://github.com/polkadot-js/apps/blob/81f9beb11092b77b188e19d53b2834705613c7e6/packages/app-rpc/src/resultToText.ts#L11
 Comment 4: This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue if you think you have a related problem or query.
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