Fixed CoroutineScope.plus operator

Fixes #559

Author: r4zzz4k

common/kotlinx-coroutines-core-common/src/CoroutineScope.kt

@@ -83,7 +83,7 @@ public interface CoroutineScope {
  * This is a shorthand for `CoroutineScope(thisScope + context)`.
  */
 public operator fun CoroutineScope.plus(context: CoroutineContext): CoroutineScope =
-    CoroutineScope(context + context)
+    CoroutineScope(coroutineContext + context)
 
 /**
  * Returns `true` when current [Job] is still active (has not completed and was not cancelled yet).

common/kotlinx-coroutines-core-common/test/CoroutineScopeTest.kt

@@ -4,6 +4,7 @@
 
 package kotlinx.coroutines.experimental
 
+import kotlin.coroutines.experimental.*
 import kotlin.test.*
 
 class CoroutineScopeTest : TestBase() {
@@ -191,4 +192,18 @@ class CoroutineScopeTest : TestBase() {
             finish(4)
         }
     }
+    
+    @Test
+    fun testScopePlusContext() {
+        assertSame(EmptyCoroutineContext, scopePlusContext(EmptyCoroutineContext, EmptyCoroutineContext))
+        assertSame(Dispatchers.Default, scopePlusContext(EmptyCoroutineContext, Dispatchers.Default))
+        assertSame(Dispatchers.Default, scopePlusContext(Dispatchers.Default, EmptyCoroutineContext))
+        assertSame(Dispatchers.Default, scopePlusContext(Dispatchers.Default, Dispatchers.Default))
+        assertSame(Dispatchers.Default, scopePlusContext(Dispatchers.Unconfined, Dispatchers.Default))
+        assertSame(Dispatchers.Unconfined, scopePlusContext(Dispatchers.Default, Dispatchers.Unconfined))
+        assertSame(Dispatchers.Unconfined, scopePlusContext(Dispatchers.Unconfined, Dispatchers.Unconfined))
+    }
+
+    private fun scopePlusContext(c1: CoroutineContext, c2: CoroutineContext) =
+        (CoroutineScope(c1) + c2).coroutineContext
 }