check for and set rate prop update in componentWillReceiveProps

[joeyscarim]

the rate prop was not updating when passing a new value to it, as referenced here: https://github.com/itsnubix/react-native-video-controls/issues/101#issuecomment-427805544

This fixes that issue, in the same way that the pause prop is handled

[kylemilloy]

I'd prefer us to remove UNSAFE functions entirely which will likely be removed in newer versions in favour of getDerivedStateFromProps or similar lifecycle methods.