Fix for get response html/text in locked body

[ferdyrurka]

Hi,

I detected a small bug during migrating from turbolinks to turbo. :)

It appears when he wants to get responseHTML or responeText on event: turbo: before-fetch-response

[dhh]

Nice. Could you add a test as well?

[ferdyrurka]

sure, tomorrow I will add tests :)

[marphi]

@ferdyrurka Does this pr resolve issue #19?

[ferdyrurka]

@marphi yes :)

I checked this use case it with the following code:

document.addEventListener('turbo:submit-end',  async (event) => {
    console.log(await event.detail.fetchResponse.responseHTML)
});