Fix extractHeaders to handle duplicate header names

The previous implementation used Seq.partition to remove ALL
headers with the matching name at once.

This commit changes its behavior to accept duplicate header names
(particularly important for some special headers such as `Set-Cookie`).

Author: gdeest

servant-client/test/Servant/ClientTestUtils.hs

@@ -138,7 +138,7 @@ type TestHeaders = '[Header "X-Example1" Int, Header "X-Example2" String]
 
 type TestSetCookieHeaders = '[Header "Set-Cookie" String, Header "Set-Cookie" String]
 
--- | AsHeaders instance for extracting two Set-Cookie headers
+-- | AsHeaders instance for extracting two headers (Required by the MultiVerbSetCookie test)
 -- Returns: (body, (cookie1, cookie2))
 instance AsHeaders '[a, b] c (c, (a, b)) where
   toHeaders (body, (h1, h2)) = (I h1 :* I h2 :* Nil, body)

servant/src/Servant/API/MultiVerb.hs

@@ -187,14 +187,16 @@ instance
     constructHeader @h x
       <> constructHeaders @headers xs
 
-  -- NOTE: should we concatenate all the matching headers instead of just taking the first one?
+  -- This implementation retrieves the *first* header with matching name.
+  -- It leaves other instances of the same header intact for subsequent extraction, which allows
+  -- multiple headers with the same name to be extracted (e.g. Set-Cookie).
   extractHeaders headers = do
     let name' = headerName @name
-        (headers0, headers1) = Seq.partition (\(h, _) -> h == name') headers
-    x <- case headers0 of
-      Seq.Empty -> empty
-      ((_, h) :<| _) -> either (const empty) pure (parseHeader h)
-    xs <- extractHeaders @headers headers1
+    idx <- Seq.findIndexL (\(h, _) -> h == name') headers
+    let (_, val) = Seq.index headers idx
+        headers' = Seq.deleteAt idx headers
+    x <- either (const empty) pure (parseHeader val)
+    xs <- extractHeaders @headers headers'
     pure (I x :* xs)
 
 class ServantHeader h (name :: Symbol) x | h -> name x where