[iOS] Button-level styles are overridden by navigator-level styles

[eliperkins]

Issue Description

If a navigator has styles set for its buttons using navBarLeftButtonFontSize, navBarLeftButtonFontFamily, etc., then styles applied with buttonFontSize, buttonFontFamily, etc. are not respected.

Steps to Reproduce / Code Snippets / Screenshots

1. Create a navigator and apply a navBarRightButtonColor: 'red' style value
2. Create a screen with a right button with the style of buttonColor: 'blue'
3. Note that the button is red and not blue.

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Environment

• React Native Navigation version: v1.1.283
• React Native version: v0.50.3
• Platform(s) (iOS, Android, or both?): iOS
• Device info (Simulator/Device? OS version? Debug/Release?): all simulators, devices, OSes, configurations