signAndSend does not respect modified signedTransaction

[valentunn]

Hey, thats Valentin from Nova Wallet

We are currently testing integration of wrapped proxy signing based on recently implemented allowCallDataAlteration
The allowCallDataAlteration itself works as expected and the payload validation logic is getting skipped. However, we noticed that an attempt to send proxy-wrapped tx is failing with "Invalid signature" response from the node
After some debugging and investigaiton we found out that despite signedTransaction is handled in signViaSigner in polkadot-js api library, polkadot-js apps itself doesnt use signAndSend which pipes signedTransaction into observeSubscribe but actually uses signAsync and send separetely, which causes signedTransaction to be lost in between since signViaSigner mutates only signature and not the modified tx itself, so send uses combination of original extrinsic and signature for the modified transaction