BUG/MINOR: cache: Cache response even if request has "no-cache" directive

BUG/MINOR: cache: Cache response even if request has "no-cache" directive

Since commit cc9bf2e5f "MEDIUM: cache: Change caching conditions"
responses that do not have an explicit expiration time are not cached
anymore. But this mechanism wrongly used the TX_CACHE_IGNORE flag
instead of the TX_CACHEABLE one. The effect this had is that a cacheable
response that corresponded to a request having a "Cache-Control:
no-cache" for instance would not be cached.
Contrary to what was said in the other commit message, the "checkcache"
option should not be impacted by the use of the TX_CACHEABLE flag
instead of the TX_CACHE_IGNORE one. The response is indeed considered as
not cacheable if it has no expiration time, regardless of the presence
of a cookie in the response.

This should fix GitHub issue #2048.
This patch can be backported up to branch 2.4.