streams: set default hwm properly for Duplex

Default highWaterMark is now set properly when using stream Duplex's
writableObjectMode and readableObjectMode options.

Added condition to the already existing split objectMode test to ensure
the highWaterMark is being set to the correct default value on both the
ReadableState and WritableState for readableObjectMode and
writableObjectMode.

Signed-off-by: Fedor Indutny <fedor@indutny.com>