React-native-navigation: iOS backButtonTitle default value is inconsistent

[nbolender]

Issue Description

The default back button title is inconsistent between iOS versions. Specifically, in iOS 8.4, the default is always "Back" while in iOS 11.2, the default is the title of the pushing screen.

Steps to Reproduce / Code Snippets / Screenshots

1. Start on a screen with a navbar title "First Title"
2. Push a screen using this.props.navigator.push(), and do not specify backButtonTitle
3. On iOS 8: back button reads "Back"; on iOS 11: back button reads "First Title"

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Environment

• React Native Navigation version: 1.1.384
• React Native version: 0.52.2
• Platform(s) (iOS, Android, or both?): iOS
• Device info (Simulator/Device? OS version? Debug/Release?): Simulator / 8.4 & 11.2 / Debug

[guyca]

@yogevbd Is this a bug? Relevant to v2 as well?

[yogevbd]

Not a bug.. This change is in iOS navBar implementation 😄

[isaachinman]

@yogevbd Is there anyway to change this? Eg to force the back button text to _always_ be "Back" instead of the previous screen's title?