converting Strings > Buffers

[mixmix]

In a few places, like where we're dealing with labels in DeriveSecret spec, we have Strings, but are combining this with a Buffer (e.g. binary format feed_id), and then passing this to hkdf.expand ...

Do we need to specify how to combine Strings and Buffers?
In JS, I convert everything to Buffers, and then in slp-encoding, we've got buffer concatenation...so it all works. I'm explicitly converting string to buffers like this:

Buffer.from('header_key', 'utf8')

i.e. specifying the encoding I'm converting from into Buffer.
Is this obvious or do we need to say it?

[mixmix]

At the very least the test vectors will surface any problem...