Fix HttpServerConnection ETag check to use If-None-Match

[94xhn]

The ETag caching in HttpServerConnection::sendResponseHeaders() returns 304 Not Modified when the If-Match request header matches the response ETag. That's the wrong header for this - If-Match is for optimistic-concurrency checks on writes (RFC 7232 §3.1), not for cache revalidation. The header a normal browser sends when revalidating a cached GET is If-None-Match, and that one isn't handled at all.

It goes a bit deeper than just checking the wrong constant, too: HttpHeaderFieldName/HTTP_HEADER_FIELDNAME_MAP in HttpHeaderFields.h never had an entry for If-None-Match in the first place. Without an entry in the table, an incoming If-None-Match header falls through to the custom-field path in HttpHeaderFields::fromString()/findOrCreate() and gets stored under its own dynamically-allocated field id, completely separate from HTTP_HEADER_IF_MATCH. So request.headers.contains(HTTP_HEADER_IF_MATCH) was always false for a request that only carries If-None-Match, and the 304 path could basically never be reached by a real client - only by something that deliberately sends If-Match on a GET, which isn't standard browser behavior.

Fix: add an IF_NONE_MATCH entry to the header field map, and switch the check in HttpServerConnection.cpp to use it instead of IF_MATCH.

I confirmed the logic by porting the field-table lookup and the 304 condition (unchanged) into a small standalone program and running both the old and new versions: with the current code, a request carrying only If-None-Match never triggers 304; with the fix it does, and a bare If-Match no longer incorrectly triggers 304 on a GET. I didn't spin up the full Sming Host build for this (submodules like FlashString aren't initialized in my checkout), so I can't point to an in-tree test run, but the header-matching code path itself is unchanged from upstream in the repro.

[what-the-diff]

PR Summary

• Introduced a New HTTP Header
  A new HTTP header field called If-None-Match has been added. This is typically used by web browsers and servers for effective cache revalidation, ensuring the optimal use of network resources by trying to avoid unnecessary data transfers.

• Modification in Conditional Check
  The existing control flow in the server connection module has been tweaked. Now, it uses the new If-None-Match header field instead of the If-Match to ascertain if a website resource has been updated. This change can enhance the data accuracy provided to the users and may reduce server load by limiting the transfer of unchanged content.