Fix for common RequestBody contentType bug. (#7340)

okhttp/src/commonTest/kotlin/okhttp3/CommonRequestBodyTest.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2022 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package okhttp3
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import kotlin.test.Test
+import okhttp3.RequestBody.Companion.toRequestBody
+
+class CommonRequestBodyTest {
+  @Test
+  fun correctContentType() {
+    val body = "Body"
+    val requestBody = body.toRequestBody(MediaType("text/plain", "text", "plain", arrayOf()))
+
+    val contentType = requestBody.contentType()!!
+
+    assertThat(contentType.mediaType).isEqualTo("text/plain; charset=utf-8")
+    assertThat(contentType.parameter("charset")).isEqualTo("utf-8")
+  }
+}

okhttp/src/nonJvmMain/kotlin/okhttp3/RequestBody.kt

@@ -40,7 +40,7 @@ actual abstract class RequestBody {
       val bytes = commonAsUtf8ToByteArray()
 
       val resolvedContentType = if (contentType != null && contentType.parameter("charset") == null) {
-        "$this; charset=utf-8".toMediaTypeOrNull()
+        "$contentType; charset=utf-8".toMediaTypeOrNull()
       } else {
         contentType
       }