Fix #29 (Add test for ErrorHandler::set)

Author: bwoebi

test/ErrorHandlerTest.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace AsyncInterop\Promise\Test;
+
+use AsyncInterop\Promise;
+
+class ErrorHandlerTest extends \PHPUnit_Framework_TestCase {
+    function testSetReturnsPreviousErrorHandler() {
+        $old = Promise\ErrorHandler::set($f = function() {});
+        $expectedF = Promise\ErrorHandler::set($old);
+        $this->assertEquals($f, $expectedF);
+    }
+}