[fix] Spread component props immutably during SSR (#8176)

By passing an empty object literal as first argument to Object.assign we can avoid having objects spread as props on a component being mutated during SSR.

Fixes #8171

src/compiler/compile/render_ssr/handlers/InlineComponent.ts

@@ -36,7 +36,7 @@ export default function(node: InlineComponent, renderer: Renderer, options: Rend
 	let props;
 
 	if (uses_spread) {
-		props = x`@_Object.assign(${
+		props = x`@_Object.assign({}, ${
 			node.attributes
 				.map(attribute => {
 					if (attribute.is_spread) {

test/runtime/samples/spread-component-immutable/_config.js

@@ -0,0 +1,15 @@
+const obj = {
+	x: 1,
+	y: 2,
+	z: 3
+};
+
+export default {
+	props: {
+		obj
+	},
+
+	test({ assert }) {
+		assert.deepEqual(obj, { x: 1, y: 2, z: 3 });
+	}
+};

test/runtime/samples/spread-component-immutable/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Widget from './Widget.svelte';
+
+	export let obj;
+</script>
+
+<Widget {...obj} x={2} />