Workaround for WFLY-3474 NullPointerException

Prior to this commit, the ServletResponseHttpHeaders.get method
would throw an NPE when used under Wildfly 8.0.0.Final and 8.1.0.Final.
This can be traced to WFLY-3474, which throws an NPE when calling
HttpServletResponse.getHeaders("foo") and that header has not
been defined prior to that.

This commit adds a check for this exception and returns null in that
case.

Issue: SPR-11919