Cache text and json after reading

[excid3]

This saves the text and json values after reading. Fetch responses don't allow you to read these twice, so it's best if we memoize the results to prevent users from running into this.

const request = new FetchRequest("post", url)
const response = await request.perform()
await response.text
await response.text
// => Failed to execute 'text' on 'Response': body stream already read

Most importantly, we need memoization so you can still read the response text after the Turbo Stream messages are read and rendered.

const request = new FetchRequest("post", url, { responseKind: "turbo-stream" })
const response = await request.perform()
await response.text
// => Failed to execute 'text' on 'Response': body stream already read

Fixes #19