core: Ignore If-Modified-Since and If-Unmodified-Since without a modification date. - #862

When a resource has no modification date (a CGI script that sends no Last-Modified, for example), ap_condition_if_modified_since() and ap_condition_if_unmodified_since() used the current time as its date. So a request with an If-Modified-Since of the current second got a 304, and any If-Unmodified-Since in the past got a 412, for content that has no date at all.

RFC 9110 13.1.3 and 13.1.4 say a recipient MUST ignore these headers if the resource does not have a modification date available. This does that: without a valid Last-Modified and without r->mtime the condition is ignored.

This is https://bz.apache.org/bugzilla/show_bug.cgi?id=39287. The same code is in 2.4.x.

I added a test in test/modules/core with a few small CGI scripts (no Last-Modified, invalid Last-Modified, and one with a valid date as a control). Without the change 5 of the 6 tests fail, with it they all pass. I also ran test/modules/core, test/modules/http1 and the pytest_suite apache, cache, negotiation, cgi and include tests before and after, with no other difference.

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