Buffer cy.visit()'s response completely before resolving

[flotwig]

Current flow

1. User issues cy.visit('http://example.com')
2. Cypress makes a request to http://example.com and buffers it internally.
3. As soon as successful headers are received and redirects are resolved, Cypress navigates the AUT iframe to load the buffered http://example.com response, which may still be streaming.

This has problems:

• it means that we can only identify errors like ECONNRESET or non-2xx error codes, but not errors like empty response bodies or timeouts while streaming the response
• it means we really can't reliably fail a test based on a cy.visit() failing

Proposed flow

1. User issues cy.visit('http://example.com')
2. Cypress makes a request to http://example.com and buffers it internally.
3. As soon as the complete response is buffered and the response ends, Cypress navigates the AUT iframe to load the buffered http://example.com response, which may not still be streaming.

This may add a small delay to the user's test; however, it should also make tests much more reliable and make it easier to diagnose cy.visit() errors.

[flotwig]

The code for this is done in cypress-io/cypress#4105, but has yet to be released.
We'll update this issue and reference the changelog when it's released.

[cypress-bot]

Released in 3.3.0.