Fix encryption feof to not return too early

Fix encryption feof to not return too early

This is because stream_read will pre-cache the next block which causes
feof($this->source) to return true prematurely. So we cannot rely on it.

Fixed encryption stream wrapper unit tests to actually simulate 6k/8k
blocks to make sure we cover the matching logic.

Added two data files with 8192 and 8193 bytes.