crash below iOS 11 when canSendWriteWithoutResponse called

[Jenus]

According to the apple docs:

The property of canSendWriteWithoutResponse is only supported iOS 11+, so we walk around this problem with the following code:

case .withoutResponse:
            if #available(iOS 11.0, *) {//bug fix for iOS 11
                return Observable<Characteristic>.deferred { [weak self] in
                    guard let strongSelf = self else { throw BluetoothError.destroyed }
                    return strongSelf.observeWriteWithoutResponseReadiness()
                        .map { _ in true }
                        .startWith(strongSelf.canSendWriteWithoutResponse)
                        .filter { $0 }
                        .take(1)
                        .flatMap { _ in
                            writeOperationPerformingAndListeningObservable(Observable.just(characteristic))
                    }
                    }.asSingle()
            } else {
                return writeOperationPerformingAndListeningObservable(Observable.just(characteristic))
                    .asSingle()
            }

[minixT]

Fixed in #243