util: fix oneshot dropping pending services immediately

[hawkw]

Motivation

Commit #330 introduced a regression when porting tower-util::Oneshot
from futures 0.1 to std::future. The intended behavior is that a
oneshot future should repeatedly call poll_ready on the oneshotted
service until it is ready, and then call the service and drive the
returned future. However, #330 inadvertently changed the oneshot future
to poll the service once, call it if it is ready, and then drop it,
regardless of its readiness.

In bthe #330 version of oneshot, an Option is used to store the
request while waiting for the service to become ready, so that it can be
taken and moved into the service's call. However, the Option
contains both the request and the service itself, and is taken the
first time the service is polled. futures::ready! is then used when
polling the service, so the method returns immediate if it is not ready.
This means that the service itself (and the request), which were taken
out of the Option, will be dropped, and if the oneshot future is
polled again, it will panic.

Solution

This commit changes the Oneshot future so that only the request lives
in the Option, and it is only taken when the service is called, rather
than every time it is polled. This fixes the bug.

I've also added a test for this which fails against master, but passes
after this change.