Set OutputStream.delegate to nil in HTTPBodyOutputStreamBridge.deinit (#46)

### Motivation

When running the cancellation tests in a loop, very occasionally there
would be a crash with the following backtrace:

```
#0	0x000000018aa008d4 in objc_opt_respondsToSelector ()
#1	0x000000018aea3410 in _outputStreamCallbackFunc ()
#2	0x000000018aea3310 in _signalEventSync ()
#3	0x000000018aeecdb0 in ___signalEventQueue_block_invoke ()
#4	0x000000018abe6cb8 in _dispatch_call_block_and_release ()
#5	0x000000018abe8910 in _dispatch_client_callout ()
#6	0x000000018abefea4 in _dispatch_lane_serial_drain ()
#7	0x000000018abf0a08 in _dispatch_lane_invoke ()
#8	0x000000018abfb61c in _dispatch_root_queue_drain_deferred_wlh ()
#9	0x000000018abfae90 in _dispatch_workloop_worker_thread ()
#10	0x000000018ad96114 in _pthread_wqthread ()
```

This seems to indicate that the output stream is trying to access its
delegate. However, when running with debug logging enabled I can see
that the delegate has already been deinitialized.

This is likely a result of the delegate itself owning the stream and
setting the stream delegate to `self`, which IIUC is an established
pattern. This presents a race in teardown.

### Modifications

This patch sets the output stream delegate to `nil` in the delegate
`deinit`.

### Result

No attempts to call the delegate will happen after it is has been
deinitailzed.

### Test Plan

With this patch, the failing test passes when run an order of magnitude
more times than were required to reliably reproduce the crash without
the patch.

Author: simonjbeaumont

Sources/OpenAPIURLSession/URLSessionBidirectionalStreaming/HTTPBodyOutputStreamBridge.swift

@@ -36,7 +36,10 @@ final class HTTPBodyOutputStreamBridge: NSObject, StreamDelegate {
         self.outputStream.open()
     }
 
-    deinit { debug("Output stream delegate deinit") }
+    deinit {
+        debug("Output stream delegate deinit")
+        self.outputStream.delegate = nil
+    }
 
     func performAction(_ action: State.Action) {
         debug("Output stream delegate performing action from state machine: \(action)")