Fixed ThreadSafeHeapTest for JDK 1.6

Author: elizarov

kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/internal/ThreadSafeHeapTest.kt

@@ -25,7 +25,7 @@ import java.util.*
 class ThreadSafeHeapTest {
     class Node(val value: Int) : ThreadSafeHeapNode, Comparable<Node> {
         override var index = -1
-        override fun compareTo(other: Node): Int = Integer.compare(value, other.value)
+        override fun compareTo(other: Node): Int = value.compareTo(other.value)
         override fun equals(other: Any?): Boolean = other is Node && other.value == value
         override fun hashCode(): Int = value
         override fun toString(): String = "$value"