In the HTTP/1.1 spec, section 13.3.4, we see:
"An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since or If-Unmodified-Since header field) and one or more entity tags (e.g., in an If-Match, If-None-Match, or If-Range header field) as cache validators, MUST NOT return a response status of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields in the request."
WSF_METHOD_HELPER .execute_existing_resource is NOT careful to do this.
nOT URGENT.