KTOR-5868 Fix the delimiter for a Cookie name-value pairs (#3739)

(cherry picked from commit 67e4f4e)

Author: marychatte

ktor-client/ktor-client-core/common/src/io/ktor/client/engine/Utils.kt

@@ -47,7 +47,8 @@ public fun mergeHeaders(
                 block(key, value)
             }
         } else {
-            block(key, values.joinToString(","))
+            val separator = if (HttpHeaders.Cookie == key) "; " else ","
+            block(key, values.joinToString(separator))
         }
     }
 

ktor-client/ktor-client-tests/common/test/io/ktor/client/tests/plugins/CookiesTest.kt

@@ -246,5 +246,17 @@ class CookiesTest : ClientLoader() {
         }
     }
 
+    @Test
+    fun testSeparatedBySemicolon() = clientTests(listOf("Js")) {
+        test { client ->
+            client.get("$TEST_HOST/encoded") {
+                cookie("firstCookie", "first")
+                header("Cookie", "secondCookie=second")
+            }.bodyAsText().also {
+                assertEquals("firstCookie=first; secondCookie=second", it)
+            }
+        }
+    }
+
     private suspend fun HttpClient.getId() = cookies(hostname)["id"]!!.value.toInt()
 }