Improve usability of StateStore to get and set new values for accounts

[lsilvs]

Expected behavior

Expect to be able to mutate the state of the account returned by store.account.get(accountId) and use it to set the new values by calling store.account.set(accountId, account)

Actual behavior

It doesn't set the new values if the account used as second parameter of store.account.set(accountId, account) is the account returned by store.account.get(accountId)

Steps to reproduce

const account = store.account.get(accountId);
account.asset = { newAttr: true };
store.account.set(accountId, account);

Which version(s) does this affect? (Environment, OS, etc...)

2.x