Merge pull request #285 from Polidea/fix/stop_scan_only_when_powered_on

API MISUSE fix

Author: bartoszstelmaszuk

Source/CentralManager.swift

@@ -163,7 +163,9 @@ public class CentralManager: ManagerType {
             return Disposables.create { [weak self] in
                 guard let strongSelf = self else { return }
                 // When disposed, stop scan and dispose scanning
-                strongSelf.manager.stopScan()
+                if strongSelf.state == .poweredOn {
+                    strongSelf.manager.stopScan()
+                }
                 do { strongSelf.lock.lock(); defer { strongSelf.lock.unlock() }
                     strongSelf.scanDisposable?.dispose()
                     strongSelf.scanDisposable = nil