When a header content is too long, the RFC demands to fold it over
multiple lines. Each line starting with a space to denote folded-lines
from regular ones.

Folding a line requires splitint it, there are only a few sweet spots
where it is possible to do so (e.g. between two words). Words are pretty
deep in the parse-tree thus multiple parts must be unwrap to reveal
them. One of those parts can be a quoted-string, printing a
quoted-string as a whole correctly wraps its content with double-quotes
but printing every child never quotes them.

When a quoted-string must be unwrap to find a sweet-splot to split the
line, we now inject double-quotes literals before and after its
children.