Fix chunk type for stream identifier

Motivation:
The problem with the current snappy implementation is that it does
not comply with framing format definition found on
https://code.google.com/p/snappy/source/browse/trunk/framing_format.txt

The document describes that chunk type of the stream identifier is defined
as 0xff. The current implentation uses 0x80.

Modifications:
This patch replaces the first byte of the chunk type of the stream identifier
with 0xff.

Result:
After this modification the snappy implementation is compliant to the
framing format described at
https://code.google.com/p/snappy/source/browse/trunk/framing_format.txt.
This results in a better compatibility with other implementations.