fix: use a shadow instead of a border for header on iOS

closes #97

Author: arnsa

src/navigators/__tests__/__snapshots__/NestedNavigator.test.tsx.snap

@@ -357,9 +357,14 @@ Array [
                           Array [
                             Object {
                               "backgroundColor": "white",
-                              "borderBottomColor": "#A7A7AA",
-                              "borderBottomWidth": 0.5,
                               "flex": 1,
+                              "shadowColor": "rgba(0, 0, 0, 0.3)",
+                              "shadowOffset": Object {
+                                "height": 0.5,
+                                "width": 0,
+                              },
+                              "shadowOpacity": 0.85,
+                              "shadowRadius": 0,
                             },
                             Object {},
                           ]
@@ -491,9 +496,14 @@ Array [
             Array [
               Object {
                 "backgroundColor": "white",
-                "borderBottomColor": "#A7A7AA",
-                "borderBottomWidth": 0.5,
                 "flex": 1,
+                "shadowColor": "rgba(0, 0, 0, 0.3)",
+                "shadowOffset": Object {
+                  "height": 0.5,
+                  "width": 0,
+                },
+                "shadowOpacity": 0.85,
+                "shadowRadius": 0,
               },
               Object {},
             ]

src/navigators/__tests__/__snapshots__/StackNavigator.test.tsx.snap

@@ -206,9 +206,14 @@ Array [
             Array [
               Object {
                 "backgroundColor": "white",
-                "borderBottomColor": "#A7A7AA",
-                "borderBottomWidth": 0.5,
                 "flex": 1,
+                "shadowColor": "rgba(0, 0, 0, 0.3)",
+                "shadowOffset": Object {
+                  "height": 0.5,
+                  "width": 0,
+                },
+                "shadowOpacity": 0.85,
+                "shadowRadius": 0,
               },
               Object {
                 "backgroundColor": "red",
@@ -519,9 +524,14 @@ Array [
             Array [
               Object {
                 "backgroundColor": "white",
-                "borderBottomColor": "#A7A7AA",
-                "borderBottomWidth": 0.5,
                 "flex": 1,
+                "shadowColor": "rgba(0, 0, 0, 0.3)",
+                "shadowOffset": Object {
+                  "height": 0.5,
+                  "width": 0,
+                },
+                "shadowOpacity": 0.85,
+                "shadowRadius": 0,
               },
               Object {
                 "backgroundColor": "red",

src/views/Header/HeaderBackground.tsx

@@ -14,8 +14,13 @@ const styles = StyleSheet.create({
         elevation: 4,
       },
       ios: {
-        borderBottomWidth: StyleSheet.hairlineWidth,
-        borderBottomColor: '#A7A7AA',
+        shadowColor: 'rgba(0, 0, 0, 0.3)',
+        shadowOpacity: 0.85,
+        shadowRadius: 0,
+        shadowOffset: {
+          width: 0,
+          height: StyleSheet.hairlineWidth,
+        },
       },
       default: {
         // https://github.com/necolas/react-native-web/issues/44