always re-run preload functions when query string changes - fixes #701

test/apps/preloading/src/routes/echo/index.svelte

@@ -0,0 +1,17 @@
+<script context="module">
+	export function preload(page) {
+		return {
+			query: page.query
+		};
+	}
+</script>
+
+<script>
+	export let query;
+</script>
+
+<pre>{JSON.stringify(query)}</pre>
+
+<a href="echo?foo=1">foo=1</a>
+<a href="echo?foo=2">foo=2</a>
+<a href="echo?foo=3">foo=3</a>