fixed willShowVisibleHeight called when hiding

The willShowVisibleHeight Driver was being called
not only when the keyboard would show but also
when the keyboard is hidden.  The added
condition ensures this isn't called when hiding.
This way the skip(1) isn't needed either.

Author: cpwhidden

Sources/RxKeyboard/RxKeyboard.swift

@@ -62,11 +62,10 @@ public class RxKeyboard: NSObject, RxKeyboardType {
     self.visibleHeight = self.frame.map { UIScreen.main.bounds.height - $0.origin.y }
     self.willShowVisibleHeight = self.visibleHeight
       .scan((visibleHeight: 0, isShowing: false)) { lastState, newVisibleHeight in
-        return (visibleHeight: newVisibleHeight, isShowing: lastState.visibleHeight == 0)
+        return (visibleHeight: newVisibleHeight, isShowing: lastState.visibleHeight == 0 && newVisibleHeight > 0)
       }
       .filter { state in state.isShowing }
       .map { state in state.visibleHeight }
-      .skip(1)
     self.isHidden = self.visibleHeight.map({ $0 == 0.0 }).distinctUntilChanged()
     super.init()