Fixed setAdapter bug

adapter should be mAdapter, otherwise when adding header view, mItemCount will get the original adapter's getCount(), which doesn't count the number of headers and/or footers. Later on in layoutChildren() method it'll throw IllegalStateException, because the number of mItemCount and mAdapter.getCount() will never be equal.
Merge pull request #85 from lzanita09/patch-1

Merged pull request #86 - Fixed setAdapter bug