CASC-223 SingleSignOutFilter requires init method to be called which changes the contract with previous versions

Problem: Some clients such as Spring Security configure the filter via Spring configuration, meaning the handler's init method
is not called vai the Filter#init method.

Solution: For now, have an atomic boolean to determine if init was called or not and call it if necessary as part of the flow.
Synchronize the init method in case we have multiple concurrent requests at the same time.
Removed no-longer valid test.
Merge pull request #72 from battags/CASC-223

CASC-223 Backwards Compatibility for Initializing Sign Out Handler in Filter