Handle case where inflateSync used when header never processed.

Handle case where inflateSync used when header never processed.

If zlib and/or gzip header processing was requested, but a header
was never provided and inflateSync was used successfully, then the
inflate state would be inconsistent, trying to compute a check
value but with no flags set. This commit sets the inflate mode to
raw in this case, since there is no other assumption that can be
made if a header was requested but never seen.