add test for object values in attributes

test/server-side-rendering/samples/attribute-escaped-quotes/_expected.html

@@ -1,3 +1,4 @@
 <div
 	foo="&#34;></div>\<script>alert(42)</script>"
+	bar="&#34;></div>\<script>alert(42)</script>"
 ></div>

test/server-side-rendering/samples/attribute-escaped-quotes/main.svelte

@@ -1,5 +1,6 @@
 <script>
 	export let foo = '"></div>\\<script>alert(42)</' + 'script>';
+	export let bar = { toString: () => '"></div>\\<script>alert(42)<\/script>' };
 </script>
 
-<div foo={foo}></div>
+<div foo={foo} bar={bar}></div>