notifications: Correctly stringify pmUsers to fix navigation to group…

… PMs.

Navigation to a group PM on pressing a notification was broken
because pmUsers was incorrectly stringified in
GroupPm.getPmUsersString. Fix, and add a test for it.

android/app/src/main/java/com/zulipmobile/notifications/FcmMessage.kt

@@ -51,7 +51,7 @@ sealed class Recipient {
      * pmUsers: the user IDs of all users in the conversation.
      */
     data class GroupPm(val pmUsers: Set<Int>) : Recipient() {
-        fun getPmUsersString() = pmUsers.sorted().joinToString { toString() }
+        fun getPmUsersString() = pmUsers.sorted().joinToString { it.toString() }
     }
 
     /** A stream message. */

android/app/src/test/java/com/zulipmobile/notifications/FcmMessageTest.kt

@@ -52,6 +52,19 @@ class FcmMessageTest : FcmMessageTestBase() {
     }
 }
 
+class RecipientTest : FcmMessageTestBase() {
+    @Test
+    fun `GroupPm#getPmUsersString gives the correct string`() {
+        expect.that(Recipient.GroupPm(pmUsers = setOf(123, 234, 345)).getPmUsersString())
+            .isEqualTo("123, 234, 345")
+    }
+    @Test
+    fun `GroupPm#getPmUsersString correctly reorders user ids`() {
+        expect.that(Recipient.GroupPm(pmUsers = setOf(234, 123, 23, 345)).getPmUsersString())
+            .isEqualTo("23, 123, 234, 345")
+    }
+}
+
 class MessageFcmMessageTest : FcmMessageTestBase() {
     object Example {
         val base = FcmMessageTestBase.Example.base.plus(sequenceOf(