[ehmicky]

This fixes using both:

• Transforms in objectMode
• The encoding option with values like hex or base64

When a transform in objectMode returns an object, encoding: 'base64' should be a noop.

When a transform in objectMode returns a string or an Uint8Array, encoding: base64 can work. However, it must operate chunk-wise instead of line-wise, since consumers of streams in objectMode might assume for emitted chunks to be independent from each other. For example, we will make that assumption with #706: we will emit one line at a time in objectMode, and would assume to be encoded separately from the next one.