fix RequestCtx is canceled (#1879)

[ksw2000]

In my opinion, the root cause of issue #1879 is that the done channel in ctx.s is nil. We can simply create done channel in fakeServer during the initialization of RequestCtx.

Original

requestCtx.Init()
→ ctx.Init2
→ ctx.s = fakeServer        (the ctx.s.done is nil)

requestCtx.Err()
→ ctx.Done()        (generate the new buffered channel and send struct{}{} since the ctx.s.done is nil)
→ error is returned

Fix

requestCtx.Init()
→ ctx.Init2
→ ctx.s = fakeServer        (the ctx.s.done is initialized)

requestCtx.Err()
→ ctx.Done()        (return ctx.s.done since it is non nil)
→ error is nil

[erikdubbelboer]

Thanks, I like it!