Add a test for the collection overload

formats/json/commonTest/src/kotlinx/serialization/EncodingCollectionsTest.kt

@@ -0,0 +1,27 @@
+package kotlinx.serialization
+
+import kotlinx.serialization.builtins.ListSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.json.*
+import kotlin.test.*
+
+class EncodingCollectionsTest {
+    object ListSerializer : KSerializer<List<String>> {
+        override val descriptor: SerialDescriptor = ListSerializer(String.serializer()).descriptor
+
+        override fun serialize(encoder: Encoder, value: List<String>) {
+            encoder.encodeCollection(descriptor, value) { index, item ->
+                encodeStringElement(descriptor, index, item)
+            }
+        }
+
+        override fun deserialize(decoder: Decoder): List<String> = throw NotImplementedError()
+    }
+
+    @Test
+    fun testEncoding() {
+        assertEquals("""["Hello","World!"]""", Json.encodeToString(ListSerializer, listOf("Hello", "World!")))
+    }
+}