KTOR-8687 Fix missing \r\n in CIO Expect-Continue response (#5019)

Author: marychatte

ktor-server/ktor-server-cio/common/src/io/ktor/server/cio/CIOApplicationEngine.kt

@@ -120,6 +120,7 @@ public class CIOApplicationEngine(
                     handleRequest(request)
                 }
             }
+
             else -> {
                 val settings = HttpServerSettings(
                     host = connectorSpec.host,
@@ -136,7 +137,7 @@ public class CIOApplicationEngine(
     }
 
     private fun addHandlerForExpectedHeader(output: ByteWriteChannel, call: CIOApplicationCall) {
-        val continueResponse = "HTTP/1.1 100 Continue\r\n"
+        val continueResponse = "HTTP/1.1 100 Continue$CRLF$CRLF"
         val expectHeaderValue = "100-continue"
 
         val expectedHeaderPhase = PipelinePhase("ExpectedHeaderPhase")
@@ -249,3 +250,5 @@ public class CIOApplicationEngine(
         }
     }
 }
+
+private const val CRLF = "\r\n"

ktor-server/ktor-server-cio/common/test/io/ktor/tests/server/cio/CIOEngineTest.kt

@@ -92,6 +92,7 @@ class CIOHttpServerTest : HttpServerCommonTestSuite<CIOApplicationEngine, CIOApp
             writePostHeaders(writeChannel, body.length)
             val continueResponse = readChannel.readUTF8Line()
             assertEquals("HTTP/1.1 100 Continue", continueResponse)
+            assertTrue { readChannel.readUTF8Line()?.isEmpty() ?: false }
 
             writePostBody(writeChannel, body)
             val response = readAvailable(readChannel)