KTOR-6183 The "charset=UTF-8" part is automatically added to the `app…

…lication/json` Content-Type (#3719)

(cherry picked from commit 240d032)

ktor-server/ktor-server-core/jvmAndNix/src/io/ktor/server/response/ApplicationResponseFunctions.kt

@@ -190,7 +190,8 @@ public suspend fun ApplicationCall.respondBytesWriter(
  * If [contentType] is `null`, it tries to fetch an already set "Content-Type" response header.
  * If the header is not available, `text/plain` is used. If [contentType] is specified, it uses it.
  *
- * Additionally, if a charset is not set for a content type, it appends `; charset=UTF-8` to the content type.
+ * Additionally, if a content type is `Text` and a charset is not set for a content type,
+ * it appends `; charset=UTF-8` to the content type.
  */
 public fun ApplicationCall.defaultTextContentType(contentType: ContentType?): ContentType {
     val result = when (contentType) {
@@ -204,10 +205,11 @@ public fun ApplicationCall.defaultTextContentType(contentType: ContentType?): Co
                 }
             } ?: ContentType.Text.Plain
         }
+
         else -> contentType
     }
 
-    return if (result.charset() == null) {
+    return if (result.charset() == null && result.match(ContentType.Text.Any)) {
         result.withCharset(Charsets.UTF_8)
     } else {
         result

ktor-server/ktor-server-tests/jvmAndNix/test/io/ktor/tests/server/http/RespondFunctionsTest.kt

@@ -5,12 +5,14 @@
 package io.ktor.tests.server.http
 
 import io.ktor.client.request.*
+import io.ktor.client.statement.*
 import io.ktor.http.*
 import io.ktor.server.application.*
 import io.ktor.server.response.*
 import io.ktor.server.routing.*
 import io.ktor.server.testing.*
 import io.ktor.util.reflect.*
+import io.ktor.utils.io.charsets.*
 import kotlin.test.*
 
 @Suppress("DEPRECATION")
@@ -69,4 +71,25 @@ class RespondFunctionsTest {
         client.get("/respond")
         client.get("/respond-with-status")
     }
+
+    @Test
+    fun testRespondWithText() = testApplication {
+        routing {
+            get("json") {
+                call.respondText("Hello", contentType = ContentType.Application.Json)
+            }
+            get("text") {
+                call.respondText("Hello", contentType = ContentType.Text.Plain)
+            }
+        }
+
+        client.get("/json").let {
+            assertEquals("Hello", it.bodyAsText())
+            assertEquals(ContentType.Application.Json, it.contentType())
+        }
+        client.get("/text").let {
+            assertEquals("Hello", it.bodyAsText())
+            assertEquals(ContentType.Text.Plain.withCharset(Charsets.UTF_8), it.contentType())
+        }
+    }
 }