Correctly mark 304 as not having a response body (#2737)

* Fixed an issue where HTTP/2 connections would disconnect if response compression were used and a 304 Not Modified response was returned

* Added HTTP encoder tests for 304 Not Modified responses

Sources/NIOHTTP1/HTTPTypes.swift

@@ -1159,6 +1159,7 @@ public enum HTTPResponseStatus: Sendable {
              .switchingProtocols,
              .processing,
              .noContent,
+             .notModified,
              .custom where (code < 200) && (code >= 100):
             return false
         default:

Tests/NIOHTTP1Tests/HTTPResponseEncoderTest.swift

@@ -117,6 +117,12 @@ class HTTPResponseEncoderTests: XCTestCase {
         writtenData.assertContainsOnly("HTTP/1.1 204 No Content\r\ncontent-length: 0\r\n\r\n")
     }
 
+    func testNoContentLengthHeadersFor304() throws {
+        let headers = HTTPHeaders([("content-length", "0")])
+        let writtenData = sendResponse(withStatus: .notModified, andHeaders: headers)
+        writtenData.assertContainsOnly("HTTP/1.1 304 Not Modified\r\n\r\n")
+    }
+
     func testNoTransferEncodingHeadersFor101() throws {
         let headers = HTTPHeaders([("transfer-encoding", "chunked")])
         let writtenData = sendResponse(withStatus: .switchingProtocols, andHeaders: headers)
@@ -153,6 +159,12 @@ class HTTPResponseEncoderTests: XCTestCase {
         writtenData.assertContainsOnly("HTTP/1.1 204 No Content\r\ntransfer-encoding: chunked\r\n\r\n")
     }
 
+    func testNoTransferEncodingHeadersFor304() throws {
+        let headers = HTTPHeaders([("transfer-encoding", "chunked")])
+        let writtenData = sendResponse(withStatus: .notModified, andHeaders: headers)
+        writtenData.assertContainsOnly("HTTP/1.1 304 Not Modified\r\n\r\n")
+    }
+
     func testNoChunkedEncodingForHTTP10() throws {
         let channel = EmbeddedChannel()
         defer {