Fix java.lang.IndexOutOfBoundsException: happens after AdapterDataObserver.onChanged()

...er.onChanged() is called)
````
java.lang.IndexOutOfBoundsException: Invalid index 6, size is 0
            at java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:255)
            at java.util.ArrayList.add(ArrayList.java:147)
            at com.eowise.recyclerview.stickyheaders.HeaderStore.isHeader(HeaderStore.java:74)
````
Another option would be to use SparseArray instead of ArrayList for isHeaderByItemPosition