Avoid an extra call to parse when the entire buffer is consumed.

The buffer offsets were reset but the the needData flag was not.
This causes an extra call to parse, the first to set the flag and
then next to read more data an attempt a parse.
Merge pull request #745 from mozilla-services/stream_parser

Avoid an extra call to parse when the entire buffer is consumed.
Merge pull request #746 from mozilla-services/versions/0.5

 Merge branch 'versions/0.5' into dev