Singular response for PATCH request (#634)

* Allow a singular entity to be returned from an update

Since a `PATCH` will be an update that could affect many rows, there might be
more than one object returned. By allowing asking for a singular response,
the/an object will be returned instead of a list. This is useful in cases when
the `PATCH` is against single entity (i.e. `/items?pkey=eq.99`).

* Add tests for plurality=singular for `PATCH` requests

* Disallow updating more than one row if `plurality=singular`

As discussed in #634, we don't want to allow updating several rows with
a `PATCH` request when the `Prefer` header specifies
`return=representation;plurality=singular` as this would almost
certainly be a client error.

* Add test for patching multiple objects with singular response

Patching > 1 object with `return=representation;plurality=singular`
should return `400 Bad Request`.

* Only add singleton range for read API requests

* Disallow inserting more than one row if `plurality=singular`

Disallow inserting several rows with a `POST` request when the `Prefer`
header specifies `return=representation;plurality=singular` as this
would almost certainly be a client error.

* Only import `q` from `Text.InterpolatedString.Perl6`

* Update OpenAPI response for PATCH/POST to mention `plurality=singular`

The OpenAPI response for PATCH/POST requests now includes the `Prefer` value
`return=representation;plurality=singular`.

* Add entry in changelog

src/PostgREST/ApiRequest.hs

@@ -190,7 +190,7 @@ userApiRequest schema req reqBody =
   endingIn xx key = lastWord `elem` xx
     where lastWord = last $ T.split (=='.') key
 
-  headerRange = if singular then singletonRange 0 else rangeRequested hdrs
+  headerRange = if singular && method == "GET" then singletonRange 0 else rangeRequested hdrs
   urlOffsetRange = rangeGeq . fromMaybe (0::Integer) $
     readMaybe =<< join (lookup "offset" qParams)
   urlRange = restrictRange