Stop using global variables as prefixes to inject the writer header.
That can cause issues when two writers set the length of the buffer in
the same header concurrently.
Stop Writing to the internal buffer twice for each write. This could
mess up with the ordering information is written.
Signed-off-by: David Calavera <david.calavera@gmail.com>
The tests added cover the case when the Writer field returns and error
and, related to that, when the number of written bytes is less than 0.
Signed-off-by: Federico Gimenez <fgimenez@coit.es>
