Allow React.Fragment for Document styles (#7863)

* Allow React.Fragment for Document styles

Fixes https://github.com/zeit/next.js/issues/7784
Closes https://github.com/zeit/next.js/pull/7860

* Fix types

* Remove `Without`

packages/next-server/lib/utils.ts

@@ -120,7 +120,7 @@ export type DocumentContext = NextPageContext & {
 }
 
 export type DocumentInitialProps = RenderPageResult & {
-  styles?: React.ReactElement[]
+  styles?: React.ReactElement[] | React.ReactFragment
 }
 
 export type DocumentProps = DocumentInitialProps & {

packages/next/pages/_document.tsx

@@ -277,7 +277,9 @@ export class Head extends Component<
         })
 
     // try to parse styles from fragment for backwards compat
-    const curStyles: React.ReactElement[] = Array.isArray(styles) ? styles : []
+    const curStyles: React.ReactElement[] = Array.isArray(styles)
+      ? (styles as React.ReactElement[])
+      : []
     if (
       inAmpMode &&
       styles &&