Wow - that getter on `HttpHeaders` has a very confusing name (getIfNotModifiedSince == if-modified-since) - so this means the `if-unmodified-since` header was always broken, right? (wrong value returned) and `if-modified-since` was always returned as a String date instead of a long.

I think the commit comment needs to mention this and I think we will need a what's new and/or migration guide entry.