Don't let new keys on `style` from `transferPropsTo` override old ones

Previous behavior: `transferPropsTo(<div style={{color: 'red'}} />)`
would get `color` overriden if we transfer in a `style={{color:
'blue'}}`. This is inconsistent with how other props are transfered.
This fixes that.

Note that this simply reverses the order of arguments for
`mergeStrategy` in `ReactPropTransferer@createTransferStrategy`. This affects `joinClasses`'s final class order but class order doesn't affect CSS
style overriding.

I could could have also created a merge that flips the argument, but I
didn't feel the need.