Clarify "Requested Variant"

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The spec uses the term "requested variant" in several places (10.2.2 201 Created, 10.2.5 204 No Content, 14.19 ETag, 14.25 If-Modified-Since, 14.28 If-Unmodified-Since). It's quite clear what it means in the context of HEAD/GET, somewhat clear for PUT, but not clear at all for other methods.

We really need to clarify this, potentially choosing a different term.

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From ​http://www.w3.org/mid/F2154EEA-1B28-4C8F-BB64-D3AD29D0F9F9@gbiv.com

  variant
     The ultimate target resource of a request after indirections
     caused by content negotiation (varying by request fields) and
     method association (e.g., PROPFIND) have been taken into account.
     Some variant resources may also be identified directly by their
     own URI, which may be indicated by a Content-Location in the
     response.

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Dependency on #109.

Part of resolving this ticket will involve removing the "requested variant" phrase from parts of the spec where its use isn't required, as part of other rewrites.

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related to #22

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Reminder: may need to revise definition of GET semantics for 200 response (p2 3.2.1) as a result of this issue's resolution.

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Basic plan is to rewrite on a case-by-case basis.

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From [856]:

Addresses #69: Clarify "Requested Variant" Addresses #109: Clarify entity / representation / variant terminology

Replaced entity with payload and variant with representation. Cleaned up description of 204 status code (related to ticket #22) Rewrote section on Content-Location and refer to def in RFC2557.

Addresses #110: Clarify rules for determining what entities a response carries

Detail the rules for Content-Location and remove the cross-ref.

Addresses #167: Content-Location on 304 responses

Removed sentence in C-Loc that refers to using C-Loc to select a

variant from cache (an already removed "feature")

Addresses #136: confusing req. language for Content-Location

Removed the confusing paragraph because HTTP does not know anything about variants behind the resource interface and thus cannot make this an interop requirement. In any case, it only existed to support the already removed cache feature that nobody implemented.

Addresses #80: Content-Location isn't special

Clarifies that C-Loc is representation metadata and what (not) to do with it if received on a request.

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From [860]:

Addresses #69: Clarify "Requested Variant"

Eliminated that phrase.

Addresses #109: Clarify entity / representation / variant terminology

Replaced variant with representation. Cleaned up p4 description of 304 and 206 status code

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Replaced with either representation, resource, or "the representation that would have been transferred in a 200 response to a GET request on the same resource", as appropriate.