fix(router): navigation should not preserve query params and fragment by default

BREAKING CHANGE

Previously both imperative (router.navigate) and declarative (routerLink) navigations
would preserve the current query params and fragment. This behavior turned out to
be confusing. This commit changes it.

Now, neither is preserved by default. To preserve them, you need to do the following:

router.naviage("newUrl", {preserveQueryParams: true, preserveFragment: true})

<a routerLink="newUrl" preserveQueryParams preserveFragment></a>

modules/@angular/router/src/url_tree.ts

@@ -183,7 +183,9 @@ export class DefaultUrlSerializer implements UrlSerializer {
   serialize(tree: UrlTree): string {
     const segment = `/${serializeSegment(tree.root, true)}`;
     const query = serializeQueryParams(tree.queryParams);
-    const fragment = tree.fragment !== null ? `#${encodeURIComponent(tree.fragment)}` : '';
+    const fragment = tree.fragment !== null && tree.fragment !== undefined ?
+        `#${encodeURIComponent(tree.fragment)}` :
+        '';
     return `${segment}${query}${fragment}`;
   }
 }