fix: don't spread error object

[kanadgupta]

Hi there — I noticed that using spread syntax on the error object strips away a lot of helpful data that we use for our assertions so this PR helps preserve that information.

You can reproduce this by opening up a Node REPL and typing in the following:

const error1 = new TypeError('one')
console.log(error1.name)
const spread = { error: { ...error1, message: 'edited' } }
console.log(spread.error.name)

Note how the name property on the error object is lost entirely.

This PR updates this logic to use Object.assign() instead — try the following in your Node REPL and observe that the error's name information is preserved:

const error2 = new TypeError('two')
console.log(error2.name)
const assigned = { error: Object.assign(error2, { message: 'edited' }) }
console.log(assigned.error.name)

Thanks again for all your work on this library!