Allow custom strategy for handling invalid cookies

[cstettler]

Currently, if a cookie cannot be decoded due to an invalid signature, the flash manager implementation returns null:

spring-cookie/src/main/java/com/innoq/spring/cookie/flash/CookieFlashMapManager.java

Line 99 in 78e8280

return null;

In some cases, instead of simply ignoring the invalid cookie, a custom behaviour should be triggered (e.g. throwing an exception).

[mvitz]

Thanks for your report.

I think this addition seems reasonable and I will try to include this.

[mvitz]

@cstettler I prepared this feature in #2. It would be nice if you could take a look and give some feedback.