Implicitly add Job in CoroutineScope constructor if it is missing

This way creating custom scopes via `CoroutineScope` is conceptually
similar to `coroutineScope { ... }` constructor.

Fixes #610

Author: elizarov

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(coroutineContext + context)
+    ContextScope(coroutineContext + context)
 
 /**
  * Returns `true` when current [Job] is still active (has not completed and was not cancelled yet).
@@ -198,7 +198,12 @@ public suspend inline fun <R> currentScope(block: CoroutineScope.() -> R): R =
     CoroutineScope(coroutineContext).block()
 
 /**
- * Creates [CoroutineScope] that wraps the given [coroutineContext].
+ * Creates [CoroutineScope] that wraps the given coroutine [context].
+ *
+ * If the given [context] does not contain a [Job] element, then a default `Job()` is created.
+ * This way, cancellation or failure or any child coroutine in this scope cancels all the other children,
+ * just like inside [coroutineScope] block.
  */
 @Suppress("FunctionName")
-public fun CoroutineScope(context: CoroutineContext): CoroutineScope = ContextScope(context)
+public fun CoroutineScope(context: CoroutineContext): CoroutineScope =
+    ContextScope(if (context[Job] != null) context else context + Job())

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

@@ -4,6 +4,7 @@
 
 package kotlinx.coroutines.experimental
 
+import kotlinx.coroutines.experimental.internal.*
 import kotlin.coroutines.experimental.*
 import kotlin.test.*
 
@@ -205,5 +206,5 @@ class CoroutineScopeTest : TestBase() {
     }
 
     private fun scopePlusContext(c1: CoroutineContext, c2: CoroutineContext) =
-        (CoroutineScope(c1) + c2).coroutineContext
+        (ContextScope(c1) + c2).coroutineContext
 }