lib-storage, imapc: Fix caching headers as empty values when handling partial header...

lib-storage, imapc: Fix caching headers as empty values when handling partial header fetches

When imapc does FETCH BODY.PEEK[HEADER.FIELDS (a b)], the result is valid
only for headers "a" and "b". The header parsing was expecting that it was
seeing the full header, so if caching decisions had remembered that also
header "c" is wanted to be cached, it was done with an empty value.