When moving back the pointer to position 0 (using stream_seek), the pointer on the encrypted stream will be moved to the position immediately after the header. Reading the header again (invoked by stream_read) will cause an error, writing the header again (invoked by stream_write) will corrupt the file. Reading/writing the header should therefore happen when opening the file rather than upon read or write. Note that a side-effect of this PR is that empty files will still get an encryption header; I think that is OK, but it is different from how it was originally implemented.
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.
