Move usage of optimistic locking 'version' out of payload

[aslakknutsen]

Optimistic locking semantics are already built into the HTTP protocol.

Return ETag header on GET

Request:

GET /api/workitems/12

Response

200
Content-Type: application/json
ETag: "0"

{ "fields": {...}}

Update object with correct v

Request:

PUT /api/workitems/12
If-Match: "0"

{ "fields": {...}}

Response

200
Content-Type: application/json
ETag: "1"

{ "fields": {...}}

Update object with wrong v

Request:

PUT /api/workitems/12
If-Match: "0"

{ "fields": {...}}

Response

412 Precondition Failed
Content-Type: application/json

Only get body if ETag has changed

Request:

GET /api/workitems/12
If-Non-Match: "0"

Response

200
Content-Type: application/json
ETag: "1"

{ "fields": {...}}

Don't return body if ETag match

Request:

GET /api/workitems/12
If-Non-Match: "1"

Response

304
Content-Type: application/json

[baijum]

I have edited the description to use workitems in the URL end points.