Return value of wallet.signTransaction is ignored

[robert-balcerowicz]

To integrate Web3Auth with Anchor I had to create a custom wallet adapter. This seemed to be a trivial task at first, however I've encountered a problem with signatures not being added to transaction. After debugging I've noticed inconsistency between TypeScript typings and actual function usage. Return value of wallet.signTransaction is ignored and this method is expected to mutate transaction object instead which is unintuitive and inconsistent with wallet.signTransactions method.