content-warning: add header considerations

[ioggstream]

I expect

to have header considerations https://tools.ietf.org/html/rfc7231#section-8.3.1
in content-warning spec

Further questions

• which is the behavior of Content-Warning in case of no-content responses eg. HEAD?

• if the "warning" attribute is part of the representation, is that supposed to be returned even by subsequent GET requests? How does this impact with caching? See Relationship with cache: are warnings returned in subsequent requests? #138

• This header SHOULD/NOT be cached/used in CONNECTION / mentioned in Vary

• Intermediaries MUST NOT modify its value. (7fb08fa)

[andrecedik]

I've added a few things to address your points:

• added section for cache considerations (2af5ba1)
• added information about/for intermediaries (7fb08fa)

I'm still working on them, so for now this is just an FYI 😉

[ioggstream]

Moved the cache/representation parte in another issue, as I think that's complex.

[andrecedik]

• Added two sections explaining, which HTTP request methods are allowed in combination with the Content-Warning field and the embedded-warning (a419ba8)

[ioggstream]

I left a comment on the above commit: a419ba8#r39174548