BUG/MINOR: http: do not ignore cache-control: public

BUG/MINOR: http: do not ignore cache-control: public

In check_response_for_cacheability(), we don't check the
cache-control flags if the response is already supposed not to be
cacheable. This was introduced very early when cache-control:public
was not checked, and it basically results in this last one not being
able to properly mark the response as cacheable if it uses a status
code which is non-cacheable by default. Till now the impact is very
limited as it doesn't check that cookies set on non-default status
codes are not cacheable, and it prevents the cache from caching such
responses.

Let's fix this by doing two things :
  - remove the test for !TX_CACHEABLE in the aforementionned function
  - however take care of 1xx status codes here (which used to be
    implicitly dealt with by the test above) and remove the explicit
    check for 101 in the caller

This fix must be backported to 1.8.