Multithreading issue on GzipChannel write header

[creyer]

When having multiple instances of WarcWriter the operations on
private static final ByteBuffer GZIP_HEADER = ByteBuffer.wrap(GZIP_HEADER_); are causing issues.
Some threads are writing the gzip header, some might not.

I think the issue could be fixed by removing the static part for the GZIP_HEADER.

[ato]

Yikes. Good catch.

Suggested fix applied in 68575d4 and released as 0.28.3.