Fix nullability on foregroundExtrapolate prop.

Author: rostislav-simonik

src/ImageHeaderScrollView.js

@@ -38,7 +38,7 @@ export type Props = ScrollViewProps & {
   renderFixedForeground: () => React$Element<any>,
   renderForeground?: () => React$Element<any>,
   renderHeader: () => React$Element<any>,
-  foregroundExtrapolate: string,
+  foregroundExtrapolate: ?string,
   renderTouchableFixedForeground?: ?() => React$Element<any>,
   ScrollViewComponent: React$ComponentType<ScrollViewProps>,
   scrollViewBackgroundColor: string,

src/index.d.ts

@@ -33,7 +33,7 @@ declare module "react-native-image-header-scroll-view" {
         renderFixedForeground?: () => React.ReactElement;
         renderForeground?: () => React.ReactElement;
         renderHeader?: () => React.ReactElement; // default is an empty view.
-        foregroundExtrapolate: string;
+        foregroundExtrapolate: ?string;
         renderTouchableFixedForeground?: () => React.ReactElement;
         ScrollViewComponent?: React.ComponentType<ScrollViewProps>;
         scrollViewBackgroundColor?: string; // defaults to white.