INT-2995: Fix HttpHeaders inconsistency

Previously, 'If-Modified-Since' and 'If-Unmodified-Since' HTTP headers were incorrectly processed
within from/to HTTP headers mapping, because Spring-Web `HttpHeaders` has a confusing method name, see:
https://jira.springsource.org/browse/SPR-10600

* fix 'If-Modified-Since' and 'If-Unmodified-Since' processing independently from `HttpHeaders`
* add fallback to formatted string for date ware HTTP headers
* add Spring Integration HTTP proxy scenario test

JIRA: https://jira.springsource.org/browse/INT-2995