Fix overwrite mechanism for web.xml atmosphere init params (#14196).

Non obvious problem before. "config" is a property of AtmosphereFramework
which also has getInitParameter(String) method. So it compiles but does
not do what is intended: to check against the user defined web.xml
servlet init-prams.
(the variableName servletConfig has the same problem :-))

Change-Id: Ib02f0b94312327d482775456c2d63a904b1e8104