Changing the push mode sometimes expires the session

[vaadin-bot]

Originally by @tomivirkki

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Changing the push mode in the Vaadin Sampler push demo (uses Vaadin 7.1 beta1) sometimes causes the user session to expire immediately

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Imported from https://dev.vaadin.com/ issue #12017

[vaadin-bot]

Originally by @Artur-

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Can't reproduce using the latest snapshot

[vaadin-bot]

Originally by @tomivirkki

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http://demo.vaadin.com/sampler/#foundation/push

[vaadin-bot]

Originally by @jouni

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Verified, I was able to get the session expired message immediately with the first try (changed from request-response to push).

[vaadin-bot]

Originally by @Artur-

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This is likely caused by Atmosphere/atmosphere#1150

[vaadin-bot]

Originally by @Artur-

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Atmosphere bug fixed. Still need to deploy a new version.