Clarification of current spec

[solderpunk]

> Since the link syntax already explicitly supports both spaces and tabs,
> IMHO it would be confusing and an invitation to implementation errors to
> explicitly define the response header as only supporting one or the
> other.  So, my vote would to be to clarify the spec by allowing both.

Since nobody objected, I've now updated the spec to clarify that the
whitespace in the response header behaves the same as the whitespace in
the link syntax.

Cheers,
Solderpunk