fix: use ElementHandle to document.body instead of document

Using `document` was causing a runtime error in the page as Playwright's
snapshot internals expect a proper element that can be "marked" via
`Element.setAttribute`.

See: https://github.com/microsoft/playwright/blob/20957f820d34a53003ab0722fa149c80fedec846/packages/playwright-core/src/server/trace/recorder/snapshotter.ts#L107-L109
See: #478

Fixes #478

lib/index.ts

@@ -157,9 +157,11 @@ function createDelegateFor<T = DOMReturnType>(
 export async function getDocument(_page?: Page): Promise<ElementHandle> {
   // @ts-ignore
   const page: Page = _page || this
-  const documentHandle = await page.mainFrame().evaluateHandle('document')
+  const documentHandle = await page.mainFrame().evaluateHandle<HTMLElement>('document.body')
   const document = documentHandle.asElement()
+
   if (!document) throw new Error('Could not find document')
+
   return document
 }