didCaptureCode called multiple times after going back in navigation controller.

[alanscarpa]

Repro steps:

• Set isOneTimeSearch to false
• Push the barcode scanner into a navigation controller
• When didCaptureCode gets called the first time, push a new view controller onto the stack.
• This works as expected up to this point and the scanner stops scanning.
• At this point, with the barcode still visible, press "Back" on the newly pushed view controller to go back to the scanner. This is where the scanner now calls didCaptureCode multiple times and pushes multiple view controllers on to the stack