Fix hasObservers conditional for Output class (#283)

The hasObservers() method should check to see if the valueRelay or the errorRelay has observers instead of checking the valueRelay twice.

rxandroidble/src/main/java/com/polidea/rxandroidble/internal/connection/RxBleGattCallback.java

@@ -353,7 +353,7 @@ private static class Output<T> {
         }
 
         boolean hasObservers() {
-            return valueRelay.hasObservers() || valueRelay.hasObservers();
+            return valueRelay.hasObservers() || errorRelay.hasObservers();
         }
     }
 }