Add tests for InvalidYieldError, improve key display

Without something like var_export, null and false are casted to an empty string.

lib/InvalidYieldError.php

@@ -12,7 +12,7 @@ class InvalidYieldError extends \Error {
         $prefix .= \sprintf(
             "; %s yielded at key %s",
             \is_object($yielded) ? \get_class($yielded) : \gettype($yielded),
-            $generator->key()
+            \var_export($generator->key(), true)
         );
 
         if (!$generator->valid()) {

test/InvalidYieldErrorTest.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Amp\Test;
+
+use Amp\InvalidYieldError;
+
+class InvalidYieldErrorTest extends \PHPUnit_Framework_TestCase {
+    public function testWithInvalidGenerator() {
+        /** @var \Generator $gen */
+        $gen = (function () {
+            if (false) {
+                yield;
+            }
+        })();
+
+        $gen->current();
+
+        $error = new InvalidYieldError($gen, "prefix message");
+        $this->assertSame("prefix message; NULL yielded at key NULL", $error->getMessage());
+    }
+
+    public function testSubgenerator() {
+        $subgen = (function () {
+            yield "foo" => 42;
+        })();
+
+        /** @var \Generator $gen */
+        $gen = (function () use ($subgen) {
+            yield from $subgen;
+        })();
+
+        $error = new InvalidYieldError($gen, "prefix");
+        $this->assertSame("prefix; integer yielded at key 'foo' on line " . (__LINE__ - 8) . " in " . __FILE__, $error->getMessage());
+    }
+}