Revalidation with matching If-Not-Modified-Since is 200 instead of 304

[SleeplessByte]

Hi there,

Currently, on revalidation, a 304 will only be generated if the precondition matches:
https://github.com/aw/CacheRules/blob/master/lib/validations.rb#L124-L127"

However, this does not handle all the preconditions:

https://tools.ietf.org/html/rfc7234#section-4.3.2

   If an If-None-Match header field is not present, a request containing
   an If-Modified-Since header field (Section 3.3 of [RFC7232])
   indicates that the client wants to validate one or more of its own
   stored responses by modification date.  A cache recipient SHOULD
   generate a 304 (Not Modified) response (using the metadata of the
   selected stored response) if one of the following cases is true: 1)
   the selected stored response has a Last-Modified field-value that is
   earlier than or equal to the conditional timestamp; 2) no
   Last-Modified field is present in the selected stored response, but
   it has a Date field-value that is earlier than or equal to the
   conditional timestamp; or, 3) neither Last-Modified nor Date is
   present in the selected stored response, but the cache recorded it as
   having been received at a time earlier than or equal to the
   conditional timestamp.

On such a request, CacheRules will return a 200 with REVALIDATED as lookup value.

The funny thing is, you have actually already written this:
https://github.com/aw/CacheRules/blob/master/lib/validations.rb#L29

A simple replacement of validator_match with precond_match here should fix this:
https://github.com/aw/CacheRules/blob/master/lib/cache_rules.rb#L89