removal of obs-fold from field-value causes inconsistencies

[reschke]

Citing Ben Kaduk (#915):

A sender MUST NOT generate a message that includes line folding
(i.e., that has any field line value that contains a match to the
obs-fold rule) unless [...]

Since we don't include the obs-fold production as a component of any
other production, and field-value excludes CRLF, it seems that any such
field line value would already be in violation of the ABNF and thus
forbidden. I don't really want to advocate for including obs-fold in
the field-value production in -semantics, though, so maybe accepting
this nit is the least bad choice here.

In RFC723x, field-value allowed obs-fold. We need to:

• tune the prose above, and
• somehow properly allow it in message/http (maybe by modifying the field-value for that specific case???)

[reschke]

see also #116