Fix isElem for Header.

The intention was obviously to ignore them, but we weren't doing so
    correctly.

servant/CHANGELOG.md

@@ -2,6 +2,7 @@ HEAD
 ----
 
 * Add `HttpVersion`, `IsSecure`, `RemoteHost` and `Vault` combinators
+* Fix safeLink, so Header is not in fact required.
 
 0.4.2
 -----

servant/src/Servant/Utils/Links.hs

@@ -166,7 +166,7 @@ type family IsElem' a s :: Constraint
 type family IsElem endpoint api :: Constraint where
     IsElem e (sa :<|> sb)                   = Or (IsElem e sa) (IsElem e sb)
     IsElem (e :> sa) (e :> sb)              = IsElem sa sb
-    IsElem sa (Header x :> sb)              = IsElem sa sb
+    IsElem sa (Header sym x :> sb)          = IsElem sa sb
     IsElem sa (ReqBody y x :> sb)           = IsElem sa sb
     IsElem (Capture z y :> sa) (Capture x y :> sb)
                                             = IsElem sa sb

servant/test/Servant/Utils/LinksSpec.hs

@@ -25,7 +25,7 @@ type TestApi =
   :<|> "get" :> Get '[JSON] ()
   :<|> "put" :> Put '[JSON] ()
   :<|> "post" :> ReqBody '[JSON] 'True :> Post '[JSON] ()
-  :<|> "delete" :> Header "ponies" :> Delete '[JSON] ()
+  :<|> "delete" :> Header "ponies" String :> Delete '[JSON] ()
   :<|> "raw" :> Raw
 
 type TestLink = "hello" :> "hi" :> Get '[JSON] Bool