Add mediator tests and clean up

core/src/main/java/com/segment/analytics/kotlin/core/platform/Mediator.kt

@@ -12,17 +12,20 @@ import kotlin.reflect.KClass
 // threads to read the list but when a modification is made the modifier is given a new copy of
 // list and that becomes the new version of the list.
 // More info: https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArrayList
-internal class Mediator(internal val plugins: CopyOnWriteArrayList<Plugin>) {
+internal class Mediator(internal var plugins: CopyOnWriteArrayList<Plugin> = CopyOnWriteArrayList()) {
 
     fun add(plugin: Plugin) {
         plugins.add(plugin)
     }
 
     fun remove(plugin: Plugin) {
-        plugins.removeAll { it === plugin } // remove only if reference is the same
+        // Note: We want to use this form of removeAll() function that takes a collection and
+        // and NOT the removeAll {} that takes a code block as that will get wrapped by MutableList
+        // and use a stateful iterator that will break when run from multiple threads.
+        plugins.removeAll(setOf(plugin))
     }
 
-    fun execute(event: BaseEvent): BaseEvent?  {
+    fun execute(event: BaseEvent): BaseEvent? {
         var result: BaseEvent? = event
 
         plugins.forEach { plugin ->
@@ -61,7 +64,7 @@ internal class Mediator(internal val plugins: CopyOnWriteArrayList<Plugin>) {
         return null
     }
 
-    fun <T : Plugin> findAll(pluginClass: KClass<T>): List<T>  {
+    fun <T : Plugin> findAll(pluginClass: KClass<T>): List<T> {
         return plugins.filter { pluginClass.isInstance(it) } as List<T>
     }
 }

core/src/main/java/com/segment/analytics/kotlin/core/platform/Timeline.kt

@@ -12,11 +12,11 @@ import kotlin.reflect.KClass
 //      Before -> Enrichment -> Destination -> After
 internal class Timeline {
     internal val plugins: Map<Plugin.Type, Mediator> = mapOf(
-        Plugin.Type.Before to Mediator(CopyOnWriteArrayList()),
-        Plugin.Type.Enrichment to Mediator(CopyOnWriteArrayList()),
-        Plugin.Type.Destination to Mediator(CopyOnWriteArrayList()),
-        Plugin.Type.After to Mediator(CopyOnWriteArrayList()),
-        Plugin.Type.Utility to Mediator(CopyOnWriteArrayList())
+        Plugin.Type.Before to Mediator(),
+        Plugin.Type.Enrichment to Mediator(),
+        Plugin.Type.Destination to Mediator(),
+        Plugin.Type.After to Mediator(),
+        Plugin.Type.Utility to Mediator()
     )
     lateinit var analytics: Analytics