feat: Add explicit headers-only POST request using Prefer header

Allows using header "Prefer=headers-only" explicitly to get a response with only a Location header.

BREAKING CHANGE: Change default for POST request from headers-only to minimal

Resolves #1656

src/PostgREST/Request/Preferences.hs

@@ -22,7 +22,7 @@ data PreferRepresentation
 instance Show PreferRepresentation where
   show Full        = "return=representation"
   show None        = "return=minimal"
-  show HeadersOnly = mempty
+  show HeadersOnly = "return=headers-only"
 
 data PreferParameters
   = SingleObject    -- ^ Pass all parameters as a single json object to a stored procedure