INT-3063: 'Expires' HTTP header workaround

According to RFC 2616 HTTP clients should ignore invalid values
for date-aware header, but Spring MVC `HttpHeaders` raises
`IllegalArgumentException` in this case.
Proposal fix to Spring Integration just to `try...catch` call of
`HttpHeaders.getExpires()` and `return null`.

JIRA: https://jira.springsource.org/browse/INT-3063