[feat] improved tests for WHATWG Response and ReadableStream by checking the response body

[jonathanong]

Describe the feature

In this PR: https://github.com/koajs/koa/pull/1830/files#diff-fd91551e086f98dc690b814777c842d122de3be3b0aa53ea726c01eeb88614aa we've added support for Blobs, ReadableStreams, and Responses. However, the tests for Response and ReadableStream do not check the response body. We should check the response body to ensure that everything is working properly.