Fix: Response.error().ok === false

[jrmann100]

Summary

• Fixes error caused in 0c1d2b9, by which Response.error().ok === true where it should be false because Response.error().status < 200.
• Adds test assertion

Explanation

Response.error internally constructs a valid Response and overwrites properties as necessary.

Now that Response must be constructed with a valid status, the constructed Response will be ok, so this property needs to be overwritten as well.