Fixed serialization of empty body

ktor-client/ktor-client-features/ktor-client-json/common/src/io/ktor/client/features/json/JsonFeature.kt

@@ -6,6 +6,7 @@ package io.ktor.client.features.json
 
 import io.ktor.client.*
 import io.ktor.client.features.*
+import io.ktor.client.features.json.JsonFeature.*
 import io.ktor.client.request.*
 import io.ktor.client.statement.*
 import io.ktor.client.utils.*
@@ -109,7 +110,8 @@ class JsonFeature internal constructor(val config: Config) {
                 context.headers.remove(HttpHeaders.ContentType)
 
                 val serializedContent = when (payload) {
-                    is EmptyContent -> config.serializer.write(Unit, contentType)
+                    Unit -> EmptyContent
+                    is EmptyContent -> EmptyContent
                     else -> config.serializer.write(payload, contentType)
                 }
 

ktor-client/ktor-client-features/ktor-client-json/common/test/KotlinxSerializerTest.kt

@@ -99,14 +99,15 @@ class KotlinxSerializerTest : ClientLoader() {
             val response = client.post<String>("$TEST_SERVER/echo-with-content-type") {
                 body = "Hello"
             }
-
             assertEquals("Hello", response)
 
             val textResponse = client.post<String>("$TEST_SERVER/echo") {
                 body = "Hello"
             }
-
             assertEquals("\"Hello\"", textResponse)
+
+            val emptyResponse = client.post<String>("$TEST_SERVER/echo")
+            assertEquals("", emptyResponse)
         }
     }
 

ktor-client/ktor-client-features/ktor-client-json/ktor-client-gson/jvm/src/io/ktor/client/features/json/GsonSerializer.kt

@@ -14,7 +14,6 @@ import io.ktor.utils.io.core.*
  * [JsonSerializer] using [Gson] as backend.
  */
 class GsonSerializer(block: GsonBuilder.() -> Unit = {}) : JsonSerializer {
-
     private val backend: Gson = GsonBuilder().apply(block).create()
 
     override fun write(data: Any, contentType: ContentType): OutgoingContent =

ktor-client/ktor-client-features/ktor-client-json/ktor-client-jackson/jvm/src/io/ktor/client/features/json/JacksonSerializer.kt

@@ -12,7 +12,6 @@ import io.ktor.http.content.*
 import io.ktor.utils.io.core.*
 
 class JacksonSerializer(jackson: ObjectMapper = jacksonObjectMapper(), block: ObjectMapper.() -> Unit = {}) : JsonSerializer {
-
     private val backend = jackson.apply(block)
 
     override fun write(data: Any, contentType: ContentType): OutgoingContent =

ktor-client/ktor-client-features/ktor-client-json/ktor-client-json-tests/jvm/src/io/ktor/client/features/json/tests/JsonTest.kt

@@ -5,17 +5,22 @@
 package io.ktor.client.features.json.tests
 
 import io.ktor.application.*
+import io.ktor.client.*
 import io.ktor.client.call.*
 import io.ktor.client.engine.cio.*
+import io.ktor.client.engine.mock.*
+import io.ktor.client.features.*
 import io.ktor.client.features.json.*
 import io.ktor.client.request.*
 import io.ktor.client.statement.*
 import io.ktor.client.tests.utils.*
+import io.ktor.client.utils.*
 import io.ktor.features.*
 import io.ktor.gson.*
 import io.ktor.http.*
 import io.ktor.request.*
 import io.ktor.response.*
+import io.ktor.response.respond
 import io.ktor.routing.*
 import io.ktor.server.engine.*
 import io.ktor.server.jetty.*
@@ -69,22 +74,45 @@ abstract class JsonTest : TestWithKtor() {
 
     protected fun TestClientBuilder<*>.configClient() {
         config {
-            install(JsonFeature) {
-                serializerImpl?.let {
-                    serializer = it
-                }
+            configJsonFeature()
+        }
+    }
+
+    private fun HttpClientConfig<*>.configJsonFeature(block: JsonFeature.Config.() -> Unit = {}) {
+        install(JsonFeature) {
+            serializerImpl?.let {
+                serializer = it
             }
+            block()
         }
     }
 
     private fun TestClientBuilder<*>.configCustomContentTypeClient(block: JsonFeature.Config.() -> Unit) {
         config {
-            install(JsonFeature) {
-                serializerImpl?.let {
-                    serializer = it
+            configJsonFeature(block)
+        }
+    }
+
+    @org.junit.Test
+    fun testEmptyBody() = testWithEngine(MockEngine) {
+        config {
+                engine {
+                    addHandler { request ->
+                        respond(request.body.toByteReadPacket().readText(), headers = buildHeaders {
+                            append("X-ContentType", request.body.contentType.toString())
+                        })
+                    }
                 }
-                block()
-            }
+                defaultRequest {
+                    contentType(ContentType.Application.Json)
+                }
+                configJsonFeature()
+        }
+
+        test { client ->
+            val response: HttpResponse = client.get("https://test.com")
+            assertEquals("", response.readText())
+            assertEquals("null", response.headers["X-ContentType"])
         }
     }