`objectMode` should remain `false` when no input streams use `objectMode`

[ehmicky]

objectMode: true is useful when streams handle other types than strings/Buffer/Uint8Array.
However, objectMode: false is more efficient with those types, as it has a higher highWaterMark and buffers them better.

When all streams passed to merge-streams use objectMode: false, the resulting stream should use objectMode: false too.

Also, it is somewhat unexpected that merging streams should change their objectMode.