Update to Atmosphere 2.1.2.vaadin5 (#13877)

New version of vaadin atmosphere, picked change from main codebase:
https://github.com/Atmosphere/atmosphere/commit/9eeb5768
Tested manually against Jetty 9.2.3

Change-Id: I90f4a90b1ff651cc475eb01c2156d3c58abd8c2c