fix iOS picker style using spread operator (bietkul#1)

Author: August Oberhauser

src/fields/picker/index.js

@@ -54,11 +54,11 @@ export default class PickerField extends Component {
     }
     return (
       <View
-        style={Object.assign(styles.pickerMainIOS, {
-          backgroundColor: theme.pickerBgColor,
-          borderBottomColor: theme.inputBorderColor,
-          borderBottomWidth: theme.borderWidth,
-        })}
+          style={{...styles.pickerMainIOS,
+              backgroundColor: theme.pickerBgColor,
+              borderBottomColor: theme.inputBorderColor,
+              borderBottomWidth: theme.borderWidth,
+          }}
       >
         <TouchableOpacity
           onPress={() => this.panel.toggle()}
@@ -100,4 +100,4 @@ export default class PickerField extends Component {
 
     );
   }
-}
+}